Volumetrically oscillating plasma flows

ABSTRACT

Volumetrically oscillating plasma flows, the volume of which controllably expands and contracts with time, are disclosed. Volumetrically oscillating plasma flows are generated by providing an energy with a power density that changes with time to the plasma-generating gas to form a plasma flow. The changes in the energy power density result in plasma flow volumetric oscillations. Volumetric oscillations with a frequency of above 20,000 Hz results in ultrasonic acoustic waves, which are known to be beneficial for various medical applications. System for providing volumetrically oscillating plasma flows and a variety of surgical non-surgical applications of such flows are also disclosed.

This application is a divisional of U.S. patent application Ser. No.12/841,361, filed Jul. 22, 2010, issuing as U.S. Pat. No. 9,089,319, thedisclosure of which is incorporated by reference herein in its entiretyfor all purposes.

1 FIELD OF INVENTION

The present invention relates to volumetrically oscillating plasmaflows. Additionally, the present invention relates to systems andmethods for generating volumetrically oscillating plasma flows and topractical applications of the volumetrically oscillating plasma flows.

2 BACKGROUND

Plasma generating devices play an important role in many areas. Forexample, plasma is used in displays, such as television sets andcomputer monitors, spectrography, in spraying applications such ascoating, and in medicine. In medicine, plasma is used for pain relief,prevention of infection spread, and surgery.

Three basic tasks that a surgeon performs during a surgery are cutting,vaporizing and coagulating tissue. Generally, cutting refers to theseparation of the tissue; vaporization refers to the controlleddestruction of tissue; and coagulation refers to the stopping ofbleeding from the tissue or blood vessels in the tissue. Most open, andeven laparoscopic, surgeries involve cutting and coagulating tissue.Some surgeries, such as the removal of tumor nodules, also involvevaporizing tissue.

The use of plasma to accomplish these three tasks is known in the art.In general, to accomplish these tasks, a plasma flow is directed at thetreated tissue, which accomplishes certain thermal effects in thetissue. For cutting and vaporization these thermal effects are thesublimation and removal of tissue. For coagulation, the desired thermaleffect is the creation of a sealing layer of necrotic tissue thatprevents further bleeding. Although plasma is considered to be asuperior way of accomplishing the three tasks, some problems with usingplasma still remain.

Presently, a surgeon typically uses a dedicated device for each of thethree surgical tasks. While this ensures that each device is welladapted to the function it is performing, switching from one task toanother requires changing devices. In a typical procedure the surgeonwill constantly need to switch from one function to the other, ascutting and vaporizing tissue exposes new bleeding tissue that must becoagulated. Changing devices during surgery adds to the duration andcomplexity of the procedure, and increases the risk to the patient. Inlaparoscopic surgery in particular, where the devices are miniaturizedand are inserted into the patient's body cavities, changing devicesfrequently is problematic. Presently, there are no known devices capableof performing all three functions well enough for a surgeon to forgo theuse of specialized devices in favor of a single, all-in-one, device.

Even the use of specialized plasma surgical devices has underlyingproblems. For example, a plasma device specifically adopted for cuttinghad to have a small outlet diameter that results in a turbulent plasmaflow suitable for cutting. The small outlet diameter makes such devicesunusable for coagulation that generally requires relatively large spotdiameter. Further, cutting of the tissue with such a device results inbleeding that not only impairs the surgeon's visibility of the treatedtissue, but, if not timely stopped, was dangerous to the patient. Asanother example, plasma devices adopted for coagulation could not stophigh-rate bleedings. Stopping even medium-rate bleeding requiressignificant experience with the device.

Accordingly, there is a need in the art for systems and methods thatallows improved control over the volumetric properties of plasma flows.In particular, there is a need for systems and methods that wouldaccomplish the three surgical tasks of cutting, vaporization, andcoagulation. Preferably, the system and method would perform the threesurgical tasks at least as well as the presently known devices.

3 SUMMARY OF THE INVENTION

This need is fulfilled by volumetrically oscillating plasma flow as wellas systems and methods for their generations. Specifically, avolumetrically oscillating plasma flow is a plasma flow in air, the flowhaving a directional axis and an active zone defined by plasma having atemperature above a threshold, wherein the active zone expands andcontracts volumetrically over time according to a controlled pattern.The volumetric oscillations process comprises controllably expanding azone of a plasma flow having a temperature above a threshold andcontrollably contracting the zone of a plasma flow having a temperatureabove the threshold the plasma flow.

A system for generating volumetrically oscillating plasma flow comprisesa power supply capable of generating an electric current having anon-zero low current level and pulses reaching a high current level (upto 50 A), wherein the pulses have a ramp rate of at least 25 A per 10μs, and a plasma-generating device capable of (1) heating aplasma-generating gas to a first temperature with the low current levelof the electric current, wherein the first temperature is at least10,000 K, (2) heating the plasma-generating gas to a second temperaturewith the high level of the electric current, wherein the secondtemperature is at least 10,000 K above the first temperature; and (3)discharging the heated plasma-generating gas as a plasma flow thatexpands in volume during the electric current pulses. Theplasma-generating device comprises an anode forming a portion of aplasma channel having an outlet with a diameter 0.3-0.8 mm. Theplasma-generating gas is supplied to the plasma-generating device at aflow rate of 0.1-0.6 L/min at room temperature. The plasma-generatingdevice may contain a cooling channel with an outlet near the outlet ofthe plasma channel capable of discharging a coolant. Additionally, theplasma-generating device may comprise a cathode assembly comprisingmultiple cathodes.

A method for generating a plasma flow whose volume varies with timeusing a plasma-generating-device having an outlet is also provided. Themethod comprises: (1) supplying a plasma-generating gas to theplasma-generating device, (2) providing an energy with a power densityto the plasma-generating gas to form a plasma flow, wherein the powerdensity changes according to a controlled pattern having a low level anda high level, and (3) discharging from the outlet of theplasma-generating device the volumetrically variable plasma flowalternating between a low intensity plasma with a temperature at theoutlet of at least 10,000 K, and a high intensity plasma with atemperature at the outlet of at least 10,000 K above the temperature ofthe low intensity plasma at the outlet, and wherein the low intensityplasma corresponds to the energy with the low level power density andthe high intensity plasma corresponds to the energy with the highintensity power density level.

For medical applications the threshold defining the active plasma zoneis about 10,000 K. A portion of the plasma when the active zone iscontracted has a first temperature of at least 10,000 K, and the portionof the plasma when the active zone is expanded has a second temperatureat least 10,000 K above the first temperature. The controlled patternmay be provided by a power supply capable of generating a current wavethat has a low current level of 3-10 A and pulses reaching a highcurrent level of 25-30 A. The ramp rate of the pulses is at least 25 Aper 10 μs. Depending on the application, the current wave may be a highfrequency biased pulse current wave with a frequency of above 2,000 Hz(preferably above 20,000 Hz) and a duty cycle of 0.05-0.15, a lowfrequency biased pulse current wave with a frequency of 20-100 Hz and aduty cycle of 0.05-0.15, or a modulated biased pulse current wave inwhich a high frequency wave is modulated by a low frequency wave with aduty cycle of the high frequency wave being 0.35-0.65 and the duty cycleof the low frequency wave being 0.05-0.15.

Applications of volumetrically oscillating plasma flows include surgicalapplications, such as the tasks of tissue cutting, coagulation andvaporization. For cutting, a high frequency biased pulse wave is usedand the operator moves the device adjacent to the tissue to make a cut.For a frequency above 20,000 Hz, the dissected tissue is coagulatedduring cutting due to the cavitation effect in the tissue created byultrasonic acoustic waves generated at this frequency. This cavitationeffect also improves blood vessel sealing. For the task of vaporization,a high frequency biased pulse wave is used. The operator moves thedevice at a distance of about 2-5 mm from the tissue. For frequenciesabove 20,000 Hz, the tissue is immediately coagulated and the vesselsare sealed as they become exposed. For the task of coagulation a lowfrequency biased pulse wave is used. The operator holds the device atthe distance of about 10-30 mm from the tissue. For improved coagulationeffect, a modulated biased pulse wave is used to create the cavitationeffect from the improved coagulation and vessel sealing. The same systemcan be used for all three surgical tasks of cutting, vaporizing andcoagulation.

Likewise volumetrically oscillating plasma flows may be used fornon-surgical applications, such as pain management, cosmetics, wastedisposal, surface cleaning and others.

4 BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A illustrates the structure of a typical plasma flow generatedwith a plasma-generating device;

FIG. 1B is a graph of the temperature of a typical plasma flow along theplasma flow axis;

FIG. 2 is a graph of the temperature along the plasma flow axis ofseveral plasma flows generated by a plasma-generating device with anoutlet diameter of 0.5 mm;

FIG. 3 illustrates the volume of active plasma for a typical plasmaflow;

FIG. 4A is a graph of the temperature of a volumetrically oscillatingplasma flow over time during a single oscillation;

FIGS. 4B-G illustrate the characteristic behavior of plasma particles atdifferent stages of a volumetric oscillation;

FIG. 5 illustrates a volumetrically oscillating plasma flow;

FIG. 6 illustrates an embodiment of a system capable of providingvolumetrically oscillating plasma flows;

FIG. 7 illustrates a longitudinal cross-section of a multi-electrodeplasma-generating device;

FIG. 8 illustrates a longitudinal cross-section of an alternativeembodiment of a multi-electrode plasma-generating device having a plasmachannel comprising an expansion portion;

FIG. 9 illustrates an arbitrary periodic current wave form forgenerating a volumetrically oscillating plasma flow;

FIG. 10 shows a power supply using three current sources from producevarious current waves for generating volumetrically oscillating plasmaflows;

FIG. 11 illustrates a biased pulse current wave;

FIG. 12 illustrates an exemplary modulated biased pulse wave;

FIG. 13A-B illustrate various current waveforms used to generatevolumetrically oscillating plasma flows;

FIG. 14A illustrates a longitudinal cross-section of an alternativeembodiment of a plasma-generating device specifically adapted forgeneration of volumetrically oscillating intermittent plasma flows;

FIG. 14B illustrates a specialized multi-cathode assembly for use in thedevice shown in FIG. 14A;

FIG. 15A illustrates single period of a voltage wave that can be appliedbetween the cathode and the anode to generate an intermittent plasmaflow;

FIG. 15B illustrates a single period of a current wave that can bepassed through the cathode and the anode to generate an intermittentplasma flow;

FIG. 15C illustrates a single period of a current wave that can bepassed through the cathode and the anode to generate intermittentvolumetrically-oscillating plasma flow;

FIG. 16 illustrates an alternative embodiment of a multi-electrodeplasma-generating device comprising pass-through cooling channels;

FIG. 17A illustrates a high frequency biased pulse current wave;

FIG. 17B illustrates a low frequency biased pulse current wave;

FIG. 17C illustrates a modulated biased pulse current wave;

FIG. 18A illustrates active plasma during a low intensity interval of avolumetrically oscillating plasma flow;

FIG. 18B illustrates active plasma during a high intensity interval of ahigh frequency volumetrically oscillating plasma flow;

FIG. 18C illustrates active plasma during a high intensity interval of alow frequency volumetrically oscillating plasma flow;

FIGS. 19A-C illustrate the differences in volume between a continuousplasma flow and the high intensity plasma of volumetrically oscillatingplasma flows with low and high frequency;

FIG. 20A illustrates a volumetrically oscillating plasma flow with arelatively short high intensity interval;

FIG. 20B illustrates a volumetrically oscillating plasma flow with arelatively long high intensity interval;

FIGS. 21A-C are graphs showing the current and dynamic pressurecomponents for a continuous plasma flow;

FIGS. 21D-F are graphs showing the current and dynamic pressurecomponents for an axially oscillating plasma flow;

FIGS. 22A-D illustrate the effect of the radial component of the dynamicpressure on the width of an axially oscillating plasma flow;

FIG. 23 is a graph of the length of the high intensity plasma flow as afunction of the period of oscillation;

FIG. 24 illustrates a surgical site created by the dissection of tissueduring surgery;

FIG. 25 illustrates a surgical site in which the blood flow has beenstopped by forming a sealing layer covering the underlying tissue;

FIG. 26A Illustrates a continuous plasma flow forming a sealing layer;

FIG. 26B is a graph of the heat flux as a function of distance from thetissue surface;

FIGS. 27A-C illustrates a rapid sublimation of tissue during coagulationwith a continuous plasma flow having a relatively high heat flux;

FIG. 28 is a graph illustrating the heat flux as a function of distancefrom the tissue surface for plasma flows of various intensities;

FIG. 29A-B illustrates the application of a single oscillation of avolumetrically oscillating plasma flow to a tissue;

FIGS. 30A-F illustrate coagulation with an axially oscillating plasmaflow over three oscillations;

FIG. 31A illustrates the surface region of a tissue at the end of thelow intensity interval of a plasma flow oscillation;

FIG. 31B illustrates the surface region of a tissue after the highintensity interval of a plasma flow oscillation;

FIG. 32 illustrates a completed sealing layer formed by an axiallyoscillating plasma flow;

FIG. 33A illustrates a surgical site covered with blood;

FIG. 33B illustrates an operator sweeping a volumetrically oscillatingplasma flow over a surgical site;

FIG. 34A-C illustrate the effect of a volumetrically oscillating plasmaflow as it sweeps across a surgical site;

FIG. 35 illustrates a variable thickness sealing layer;

FIG. 36 is a graph of the temperature of a volumetrically oscillatingplasma flow along the plasma flow axis during both the high intensityinterval and the low intensity interval;

FIG. 37 illustrates an operator sweeping a continuous plasma flow over asurgical site;

FIG. 38 is a graph of the vaporization heat flux plotted for threedifferent bleeding rates as a function of the high intensity interval;

FIG. 39 is a graph of the short time approximation and the numericalsimulation of compact layer formation as a function of the low intensityinterval;

FIG. 40 is a graph of the rate of compact layer formation as a functionof the low intensity interval plotted based on experimental data;

FIG. 41 is a graph of the compact layer thickness as a function of thenumber of oscillations for several different frequencies;

FIG. 42 illustrates a partially coagulated surgical site with an exposedblood vessel;

FIGS. 43A-C illustrates the application of axially oscillating plasma toseal a blood vessel;

FIG. 44 is a graph of the radius of a bubble as a function of time underthe influence of ultrasonic cavitation;

FIGS. 45A-C illustrate the process of cutting with a typical prior artcontinuous plasma flow;

FIGS. 46A-C illustrate the process of cutting with a radiallyoscillating plasma flow;

FIG. 47 illustrates the radial component of a radially oscillatingplasma flow sealing a blood vessel;

FIGS. 48A-E illustrate the vaporization of a tumor with a volumetricallyoscillating plasma flow;

FIG. 49A-B illustrate a volumetrically oscillating plasma flow sealing ablood vessel exposed during vaporization;

FIG. 50 illustrates a volumetrically oscillating plasma waste disposalsystem.

5 DETAILED DESCRIPTION OF THE EMBODIMENTS 5.1 Introduction toVolumetrically Oscillating Plasma

By way of introduction, a plasma flow is a stream of gas particles inwhich a non-negligible number of the gas particles are ionized. Onecommon way of generating a plasma flow is to heat a stream of gas,referred to as the plasma-generating gas, to a high enough temperatureto ionize a portion of the gas particles. FIG. 1A illustrates alongitudinal cross-section of a typical plasma flow generated in thismanner. Plasma flow 1 is discharged by a plasma-generating devicecomprising tip 2 having an outlet 3. Plasma flow 1 propagates away fromtip 2 along plasma flow axis 4. Under certain flow conditions, plasmaflow 1 remains laminar and does not mix significantly with surroundingmedium 5, which is typically air. A laminar flow, such as plasma flow 1,is characterized by a high concentration of energy in the core of theflow, i.e., at, or in close proximity to, axis 4, and a rapid radialtemperature drop off. Typically the distribution of energy andtemperature in plasma flow 1 transverse to plasma flow axis 4 issubstantially parabolic.

A plasma flow with a substantially parabolic temperature distributiontransverse to plasma flow axis 4 does not have a single temperature. Formany purposes, however, it is useful to characterize the plasma flowwith a single representative temperature. One way to characterize thetemperature of the plasma flow is to consider the temperature in thecore of the flow. Another way to characterize the temperature of theplasma flow is to consider the average temperature in the flow at agiven cross section. The term “outlet temperature” and its variationsrefer to a representative temperature, preferably the core temperature,of the plasma flow as it is discharged through outlet 3.

The temperature of plasma flow 1 decreases as it propagates along axis 4away from outlet 3. Plasma flow 1 has a proximal region 6 with atemperature close to the outlet temperature of the plasma flow as it isdischarged from outlet 3. Plasma flow 1 also has a distal region 7 witha lower temperature. Because there is little mixing with surroundingmedium 5, a laminar plasma flow maintains a substantially uniformtemperature in region 7 over a significant distance away from outlet 3.FIG. 1B shows a plot of the temperature of the plasma flow along plasmaflow axis 4. Region 10 and region 11 in FIG. 1B correspond to thetemperatures found in proximal region 6 and distal region 7 in FIG. 1A,respectively. As seen from FIG. 1B, the plasma flow temperaturemaintains a relatively constant temperature over region 10. At distance9 from the plasma-generating device, the plasma flow temperaturedecreases significantly. In region 11, the plasma flow maintains arelatively constant temperature. Distance 9, at which the plasma flowexperiences the most significant temperature drop off, is considered ademarcation point between the proximal region 6 and distal region 7.

FIG. 2 shows a series of plasma temperature measurements taken at avarious distances from a plasma-generating device. The diameter ofoutlet 3 used for these measurements was 0.5 mm. Each line in FIG. 2represents a different outlet temperature. Each line clearly exhibits aproximal region and distal region as described above. As seen from FIG.2, as the outlet temperature of the plasma flow increases, the proximaland distal regions extend over larger distances away from outlet 3 ofthe plasma-generating device.

There is no clear boundary of the plasma flow. For the purposes of thisdisclosure, the discussions of volumetric plasma oscillations should beunderstood as volumetric oscillations of “active plasma.” In thisdisclosure, active plasma is defined as plasma with a temperature abovecertain threshold of interest. For example, for surgical applications,such threshold may be 10,000K. For some industrial applications, thisthreshold may be higher, while for cosmetic applications this thresholdmay be lower. Plasma with a temperature lower than a given threshold,outside the active plasma boundary, still may have beneficial effects.This, in this disclosure, “active plasma” provides a way to convenientlydescribe the volume (or zone) of a plasma flow, but is not meant todelimit the portion of what may be useful plasma. For FIG. 2, theillustrated active plasma threshold is 10,000 K.

In reference to the size of a plasma flow, for purposes of thisdisclosure, the term “volume” is defined as the space occupied by activeplasma. When the plasma flow discharged from outlet 3 has a relativelyhigh temperature, the plasma in both proximal region 6 and distal region7 may contain active plasma. Accordingly, the volume of such a plasmaflow would span both of these regions. For example, in FIG. 2 the curvecorresponding to a plasma flow with an outlet temperature of 15,000 Kshows a distal region with a temperature of approximately 12,000 K. Fora plasma flow with a relatively low outlet temperature, for example,approximately 12,000 K, only the proximal region contains active plasma.In this case, the volume of the plasma flow is significantly smaller.Plasma flows with an outlet temperature of less than the active plasmathreshold by definition have no active plasma and no volume.

The terms “length” and “width,” with reference to the plasma flow aredefined in a similar manner as “volume.” The “length” of a plasma flowis the distance between outlet 3 and the point along plasma flow axis 4where the temperature drops below 10,000 K. FIG. 3 shows a plasma flowwith active plasma volume 20. The length of this plasma flow is thedistance between outlet 3 and point 21 along plasma flow axis 4. The“width” of a plasma flow at a given distance from outlet 3 is defined asthe diameter of a cross-section of the active plasma flow transverse toplasma flow axis 4. For example, the plasma flow in FIG. 3 has a maximumwidth in plane 22, which is the diameter of the region in plane 22occupied by active plasma. The term “spot diameter” refers to the widthof the plasma flow at the spot where it comes in contact with asubstrate, such as a tissue.

In a preferred embodiment, volumetrically oscillating plasma isgenerated with a plasma-generating device with annular components. Inthis embodiment, because outlet 3, and as a result a cross-section ofthe plasma flow, is circular, the width of the plasma flow issubstantially the same at any angle in the cross-section. In otherembodiments, the width of the plasma flow may differ when measured atdifferent angles. In those embodiments, for notational convenience, thewidth of a plasma flow is defined as the largest cross-sectionaldiameter of the active plasma transverse to plasma flow axis 4.

A “volumetrically oscillating plasma flow,” as the term is used herein,refers to a flow of plasma whose volume varies in time by expanding andcontracting. Preferably these volumetric variations are controlled. Formedical applications, for example, the greatest benefit is achieved whenthe plasma volume varies according to a periodic pattern.

A volumetrically oscillating plasma flow can be created by providing tothe plasma-generating gas energy with a power density that oscillates intime between a low level during a low intensity interval and a highlevel during a high intensity interval. Providing energy with a lowlevel power density to the plasma-generating gas results in thegeneration of low intensity plasma, while providing energy with a highlevel power density results in generation of high intensity plasma.

Additional energy provided to the plasma-generating gas during the highintensity interval, as compared to the low intensity interval, resultsin an increase of the plasma flow temperature. In a preferredembodiment, energy is supplied by passing an electric current throughthe plasma-generating gas as it flows through a plasma-generatingdevice. In alternative embodiments the energy may be supplied to theplasma-generating gas using microwaves or by means of electromagneticfields as known in the art. The plasma flow generated by an energy withoscillating power density, provided to the plasma-generating gas, has avolume that oscillates in time with the same frequency as the energy.

An exemplary current waveform that meets the criteria of supplying a lowpower density level and a high power density level to theplasma-generating gas is a biased pulse wave, in which the current isbiased at a low level, referred to as the “bias level” and has pulsesreaching a high level, referred to as the “pulse level.” FIGS. 4A-Fillustrate the behavior of the plasma particles at different timesduring a single period of a biased pulse wave volumetric oscillation.FIG. 4A shows the temperature of the plasma as a function of time,starting at a low temperature corresponding to a low intensity plasmaduring a time interval corresponding to the bias current, then risingrapidly to a high temperature corresponding to a high intensity plasmaduring a time interval corresponding to the pulse current, and thenreturning rapidly to the low temperature corresponding to the lowintensity plasma during the time interval corresponding to the next biascurrent. The characteristic behavior of the particles at the stagesshown in FIG. 4A is shown in FIGS. 4B-F, respectively.

As shown in FIG. 4B, relatively slow particles 71, corresponding to thelow intensity plasma, travel along the plasma flow axis. Particlevelocities are shown by their associated velocity vectors. Due to arelatively low temperature, particles 71 have a relatively high density.As shown in FIG. 4C, as the temperature of the plasma flow rapidlyincreases at the beginning of the high intensity interval, particles 73accelerate to a higher velocity than the velocity of particles 71. Dueto a relatively high temperature, particles 73 have a relatively lowdensity. These sparse, fast-moving particles 73 travel at a high speedand quickly catch up to dense, slow-moving particles 71 downstream inthe plasma flow. As shown FIG. 4D, collisions between the sparse,fast-moving particles 73 and the dense, slow-moving particles 71 causeplasma particles to scatter. Scattered particles 74 now have componentsto their velocity vectors in both the radial and axial directions. Thisscattering causes the width of the plasma flow to increase, marking thebeginning of a radial oscillation. This process is analogous to a singlebilliard ball hitting a group of billiard balls, all scattering indifferent directions after the hit.

At a certain time, referred to as transition time t_(transition), thefast-moving low density particles of the high intensity plasma havepushed away all of the low intensity, high density particles of the lowintensity plasma. Thereafter, the fast-moving low density particles ofthe high intensity plasma propagate unimpeded along the plasma flowaxis. At this time, shown in FIG. 4E, the plasma flow length begins toincrease, marking the beginning of an axial oscillation.

In FIG. 4F, with the drop in temperature, caused by the drop in thecurrent at the end of the pulse, the velocity of the particlesdischarged from the device drops and the density of the plasmaincreases. This creates pressure gap 75 because slow-moving particles 71of the next low intensity interval do not catch up with the fast-movingparticles 73.

If the current is dropped to the bias level before transition timet_(transition)) the length of the plasma flow does not have anopportunity to increase. In this situation, the plasma flow would onlyundergo the processes shown in FIGS. 4B, 4C, 4D, and 4G. Therefore, apulse of current that is shorter than transition time t_(transition)produces a predominantly radial oscillation, while a pulse of currentthat is longer than transition time t_(transition) produces apredominantly axial oscillation that is preceded by a single radialexpansion. Volumetric oscillations are produced by repeated increasesand decreases in the supplied current. In this disclosure the term“radially oscillating plasma flow” and its derivatives refer to avolumetrically oscillating plasma flow with radial oscillations andsmall-scale axial oscillations, i.e., oscillations that do not exceedone order of magnitude the size of the outlet diameter. The term“axially oscillating plasma flow” and its derivatives refers to avolumetrically oscillating plasma flow with predominantly large-scaleaxial oscillations, i.e., oscillations that exceed one order ofmagnitude the size of the outlet diameter. It should be understood,however, that axially oscillating plasma flow starts with a radialexpansion.

Another way to describe a volumetrically oscillating plasma flow is byobserving its behavior in a space with a constant volume. In this fixedspace, a portion of the space is occupied by plasma and the remainingportion is occupied by the surrounding medium. Such medium is typicallyair, but may be another gas, or even a liquid for example in certainlaparoscopic surgeries or even underwater applications. For avolumetrically oscillating plasma flow, the portion of the spaceoccupied by plasma oscillates in time. Conversely, the portion of thespace occupied by the surrounding medium also oscillates. In the fixedspace, during a low intensity interval, plasma occupies a smallerportion of the space than during a high intensity interval. In apreferred embodiment, the low intensity plasma has a portion with atemperature of at least 10,000 K and the high intensity plasma has aportion with a temperature at least 10,000 K above that of the lowintensity plasma temperature. It should be noted that the portion of thespace occupied by plasma and the surrounding medium are not necessarilycontiguous. Plasma particles may be dispersed through the surroundingmedium as shown in FIG. 5. Further, because the densities of plasma andthe surrounding medium may be vastly different, certain methods ofvolume computation should preferably take the density intoconsideration.

An alternative way of characterizing a volumetrically oscillating plasmaflow is by expansion and contraction of active plasma. In a preferredembodiment the expansion and contractions are according to a controlledpattern.

Volumetrically oscillating plasma flows also produce acoustic waves.Briefly, at the time of plasma flow expansion, when plasma temperaturerapidly rises, air is displaced by the expanding plasma flow. At thetime of plasma flow contraction, when the plasma temperature rapidlydrops, the air is pulled into the resultant low pressure region that wasjust occupied by the fast-moving plasma particles. When the plasma flowoscillates repeatedly, these air movements form an acoustic wave.

Volumetrically oscillating plasma flows are useful in a variety ofapplications, for which the use of continuous plasma flows, producedwith constant energy supply, is not ideal. The illustrative applicationsin this disclosure are in the medical field. Other applications include,among others, the treatment of electronic components, cosmetics, andwaste disposal applications.

5.2 System

Referring to FIG. 6, an embodiment of a system capable of providing avolumetrically oscillating plasma flow generally comprises console 31and plasma-generating device 32. In a preferred embodiment, console 31provides energy in the form of an electric current, plasma-generatinggas, and coolant to plasma-generating device 32 through connector 33.Console 31 preferably has control circuitry, such as a processor, foroperating the plasma-generating device 32 and a user interface comprisedof a display and controls. An operator programs the mode of operation ofthe system with the controls of console 31 in accordance with parametersfor a given application, then uses plasma-generating device 32 todischarge a plasma flow.

In a preferred embodiment specifically adopted for surgical use,plasma-generating device 32 is a hand-held surgical device. The operatormay be a trained medical professional, such as a surgeon.Plasma-generating device 32 may be adapted for performing open orlaparoscopic surgery.

FIG. 7 shows a longitudinal cross-section of a multi-electrodeplasma-generating device 41 suitable for generating volumetricallyoscillating plasma flows. Plasma-generating device 41 comprises anode42, a cathode 43, and a number of intermediate electrodes 46′, 46″,46′″. Depending on the application, the number of the intermediateelectrodes may vary. Together with anode 42, intermediate electrodes46′, 46″, 46′″ form a plasma channel. The cross-sectional diameter ofthis plasma channel may vary with distance from the cathode. FIG. 7shows a plasma channel comprising two separate portions with differentcross-sectional diameters. Plasma channel heating portion 58 is formedby intermediate electrodes 46′, 46″, and 46′″. Anode 42 forms plasmachannel anode portion 45. In this embodiment the cross-sectionaldiameter of plasma channel heating portion 58 is slightly smaller thanthat of the plasma channel anode portion 45. In the preferredembodiment, the cross-sectional diameter of plasma channel anode portion45 corresponds to the diameter of outlet 3, which is one of the keyparameters controlling the properties of the generated plasma flow.Intermediate electrodes 46′, 46″, 46′″ and anode 42 are separated fromdirect contact with each other by annular insulators 47′, 47″, 47′″.Sleeve 44 forms gas supply channel 59 that runs along cathode 43 intothe cathode chamber 49 that hosts tip 48 of cathode 43.

During operation, the plasma-generating gas is supplied intoplasma-generating device 41. Once in the plasma-generating device theplasma-generating gas is passed to plasma chamber 49 and then intoplasma channel heating portion 58 through gas supply channel 44. Fromthere, the plasma-generating gas enters plasma channel anode portion 45.After traversing plasma channel anode portion 45, the plasma-generatinggas is discharged through outlet 3. The plasma-generating gas ispreferably argon. Alternatively, an inert gas or air may be used as theplasma-generating gas. A plasma flow is generated by heating theplasma-generating gas as it passes through plasma channel heatingportion 58. In the preferred embodiment, the energy for heating theplasma flow in plasma channel heating portion 58 is transferred to theplasma-generating gas by an electric arc established between cathode 43and anode 42. This electric arc is generated by passing a current fromconsole 31 between cathode 43 and anode 42.

For a plasma flow formed with an electric current arc discharge, thetemperature of the plasma flow depends on the current of the electricarc, the plasma-generating gas flow rate, and the diameter of heatingportion 58. The temperature of the plasma flow as it leaves heatingportion 58 is proportional to the ratio of the current to the diameterof heating portion 58, i.e.,

${T \propto \frac{I}{d_{heating}}},$where T is the temperature in K, I is the current in A, and d_(heating)is the diameter of heating portion 58 in mm. As seen from the aboverelationship, high temperatures can be achieved by passing a highcurrent through a plasma-generating device with a small heating portiondiameter. Electrodes 46 and anode 42 are composed of presently known inthe art materials that can safely sustain a continuous temperature of12,000-13,000 K. Exceeding this temperature increases the risk ofelectrode erosion, which is unacceptable for medical applications inwhich plasma impurities may be harmful to the patient. It was observed,however, that increasing the temperature up to 30,000 K temporarily, forup to several milliseconds, does not result in electrode erosion.Temporary increases of the current passed between cathode 43 and anode42 up to 30 A were found to not cause erosion and to be otherwise safe,while increasing the plasma flow temperature to 20,000-30,000 K. Toachieve 30,000 K with this current, the diameter of heating portion 58should be 0.6 mm or less, and is preferably 0.3-0.5 mm.

The plasma is discharged from outlet 3 of the plasma-generating device.The preferred outlet diameter varies based on the application. Forexample, medical applications such as tissue cutting require a smallwidth plasma flow to achieve precise cuts. Referring again to FIG. 7,this embodiment shows a plasma-generating device where the diameter ofthe plasma channel anode portion 45 and the diameter of outlet 3 are notmuch larger than the diameter of plasma channel heating portion 58. Inthe preferred embodiment, the diameter of outlet 3 is about 0.3-0.8 mm,preferably 0.5 mm.

For some applications, an outlet with a large diameter is preferred. Alarge outlet diameter allows generating a plasma flow with a large spotdiameter. Such a large spot diameter is preferred for certainapplications such as wound healing, cosmetics, and cleaning. Someapplications may not require the volumetrically oscillating plasma tohave very high outlet temperatures. But for those applications thatactually require high outlet temperatures and a large spot diameter, anembodiment as illustrated in FIG. 8 may be used.

As shown in FIG. 8, the diameter of the plasma flow may be graduallyexpanded within the device. FIG. 8 shows an embodiment of theplasma-generating device with a plasma channel having an expansionportion. The plasma channel comprises heating portion 58, expansionportion 57, and anode portion 45. Expansion portion 57 comprises one ormore expansion sections. In the embodiment shown in FIG. 8, expansionportion 57 comprises expansion sections 54, 55, 56. Starting at heatingportion 58, the diameter of each successive expansion section increases,terminating with anode portion 45 with the desired outlet diameter forthe particular application. To guarantee proper expansion of the plasmaflow in the plasma channel, in the context of the examples discussedabove each successive section should preferably increase in diameter byabout 0.2-0.6 mm, and have a length between approximately one and twotimes its diameter.

In some embodiments of the plasma-generating device, heating portion 58and each section of expansion portion 57 are formed by separateelectrodes. In other embodiments of the plasma-generating device, asingle intermediate electrode may form portions of two or more adjacentsections. In yet some other embodiments, some intermediate electrodesmay form a portion of a section, or an entire section, of the heating orexpansion portions, and other intermediate electrodes may form onlyportions of two or more adjacent sections. In the embodiment shown inFIG. 8, intermediate electrode 46′ forms a cathode chamber and a part ofthe heating portion, intermediate electrode 46″ forms expansion section54 (and part of heating portion 58), and intermediate electrode 46′″forms expansion sections 55 and 56.

Referring back to FIG. 6, in the preferred embodiment, console 31contains control circuitry capable of causing the power supply todeliver an electric current suitable for generating a volumetricallyoscillating plasma flow. The current is transmitted to plasma-generatingdevice 41 through connection 33 to console 31. The control circuitry ispreferably capable of causing the power supply to provide an arbitraryperiodic current wave, such as wave 54 shown in FIG. 9. The basiccharacteristic of current 54 is repetition of a desired waveform withperiod τ. Current 54 oscillates between a low-level current I_(L), andhigh-level current I_(H). This arbitrary current waveform may havefeatures that vary slowly with time such as smooth region 55 or rapidlysuch as rectangular pulse 56. A variety of periodic current waves may besuitable for generating a volumetrically oscillating plasma flow.

As discussed above, volumetric oscillations of a plasma flow are causedby changes in the plasma flow temperature, which alters the length,width, and volume of the plasma flow. Particularly for radialoscillations, the magnitude of these changes depends on how rapidly theplasma temperature changes. This is due to the scattering mechanismdescribed above in connection with FIG. 4D, which is more pronouncedwhen the differences in density and velocity of the particles aresignificant. For example, if current 54 varies slowly, the density andvelocity of the particles change only gradually and the amount of radialscattering is small.

In a preferred embodiment, the control circuitry is capable of causingthe power supply to generate rectangular pulses. In this embodiment, thepower supply uses three current sources, as schematically shown in FIG.10. The first current source is capable of providing a constant biascurrent. The second current source is capable of providing rectangularpulses with a frequency of about 20-100 Hz. Preferably the secondcurrent source has a ramp rate of 25 A per 10 μs. The third currentsource is capable of generating rectangular pulses with a frequency ofabout 20,000-100,000 Hz. The third current source has a ramp rate of atleast 20 A per 1 μs, preferably more 25 A per 1 μs. The power supplypreferably generates up to 50 A. A higher current which may beunacceptable for medical applications, however, may be preferred forindustrial applications.

For many applications it is desirable to introduce rapid variations inthe current to produce radially or axially oscillating plasma flows.FIG. 11 shows a biased pulse wave capable of producing suchoscillations. In this preferred embodiment, the oscillating current usedto generate a volumetrically oscillating plasma flow maintains the biascurrent level, coupled with periodic pulses reaching the pulse currentlevel. A single period 51 of current 50 may be viewed as the highintensity interval 52 and low intensity interval 53. During lowintensity interval 53, the current is maintained at bias current levelI_(L). During high intensity interval 52 the current is raised to pulsecurrent level I_(H). Low intensity interval 53 and high intensityinterval 52 are maintained for times 53 and 52, respectively. The dutycycle D of current 50 is given by the ratio of high intensity interval52 t_(H) and the period of the oscillation, t_(H)+t_(L) as

$D = {\frac{t_{H}}{t_{H} + t_{L}}.}$The frequency f of current 50 oscillations is

$f = {\frac{1}{t_{H} + t_{L}}.}$

As explained above in connection with FIGS. 4A-G, a biased pulse currentwave, such as the one shown in FIG. 11, produces a volumetricallyoscillating plasma flow that can be radially or axially oscillatingdepending on the frequency. Significant radial oscillations, forexample, can be generated when the current has a frequency greater than2,000 Hz, preferably in the range of 20,000-30,000 Hz. This frequencyensures that the duration of the pulse current level is close to thetransition time of the plasma flow, which does not provide sufficienttime for a significant axial oscillation to develop.

For generation of predominantly axial oscillations, the frequency of thebiased pulse wave is preferably 20-100 Hz. This frequency range ensuresthat the duration of the high intensity interval is longer than thetransition time, so that the plasma flow has the opportunity to expandin length during the pulse. Regardless of the frequency used, however,the average current is determined by the duty cycle D of the biasedpulse wave. In general, for volumetrically oscillating plasma flows theaverage current must be kept at a level low enough to avoid damagingcomponents of the plasma-generating device. At the same time, to createa significant increase in temperature of the plasma flow, pulse currentlevel, I_(H), must be relatively high. The operational bias currentlevel, I_(L), is 3-10 A, preferably around 6 A, and the pulse currentlevel, I_(H), is around 25-30 A, with the duty cycle D of 0.05-0.15.These preferred settings produce an average current level of only7.2-9.6 A, while alternating between a low temperature of 11,000 K and ahigh temperature of 20,000-30,000 K. The brief current increases to 30 Awere found not to cause damage to the plasma-generating devicecomponents.

In some embodiments the control circuit is capable of generating amodulated biased pulse wave shown in FIG. 12. In the wave shown in FIG.12 high frequency pulses are modulated by low frequency pulses. Thiscurrent wave combines the effects of both low frequency and highfrequency biased pulse waves. This current wave is structured like a lowfrequency biased pulse wave, except that the constant pulse currentlevel during the high intensity interval has been replaced by a highfrequency biased pulse wave. Preferably, for the modulated pulse wave,the high-frequency frequency is 20,000-30,000 Hz, high frequency dutycycle is 0.35-0.65, low-frequency frequency is 20-100 Hz, low frequencyduty cycle is 0.05-0.15, bias current level is 3-10 A, and pulse currentlevel is 25-30 A. In one embodiment of the modulated biased pulse wave,shown in FIG. 12, the low frequency period τ₁ is 30 ms and the lowfrequency duty cycle D₁ is 0.13. The high frequency period τ₂ is 40 μmand the high period duty cycle D₂ is 0.5. When such a current wave isused to generate a volumetrically oscillating plasma flow the resultingplasma flow exhibits significant axial oscillations coupled with highfrequency radial oscillations during the high intensity interval. Thelarge high frequency duty cycle D₂ ensures that the high frequencyoscillations do not limit the length of the plasma flow during the highintensity interval.

Instead of a rectangular low frequency pulse, other envelope functionscan be used. FIGS. 13A and 13B show examples of such waves. In FIG. 13A,a sine wave is used as an envelope for the high frequency pulses. InFIG. 13B, a chirp wave is used as an envelope of the low frequencypulse. Using a non-constant envelope for the high frequency pulseprovides amplitude modulation of the current wave during the highintensity interval. Further, the bias current level does not need to beconstant. In some embodiments, the bias current level changes in time.All electric current waveforms that are disclosed in connection with thepreferred embodiments may provide energy to the plasma-generating gaswhich may alternatively be provided by other means such as magneticfield or microwaves.

Device 32 may be also be adapted for generating an intermittent plasmaflow. An intermittent plasma flow is a flow that is heated by anelectric arc periodically. The electric arc is periodically turned ‘on’and ‘off.’ During the ‘on’ state the arc between cathode 43 and anode 42is established and maintained. During the ‘off’ state the arc isextinguished and room-temperature plasma-generating gas is dischargedfrom outlet 3. An intermittent plasma flow, in which the plasma flowceases, should not be confused with a volumetrically oscillating plasmaflow, in which the plasma volume increases and decreases. To produceintermittent plasma, the plasma-generating device has to overcome twoproblems. First, during the startup of the ‘on’ state, electrodes erodecreating impurities introduced into the plasma flow. While this does notpresent a problem for a non-intermittent plasma flow in which thestartup occurs once for the entire procedure, for the intermittentplasma, the generated impurities for each ‘on’ period may render thedevice unsuitable for medical use. This problem is overcome by the useof a special startup current sequence before the current is ramped tothe operational level during the ‘on’ state as shown in FIG. 15B.Second, the area of electric arc attachment to the cathode grows witheach subsequent ‘on’ state until it includes the cathode holder. Whenthe area of the electric arc attachment reaches the cathode holder, thecathode holder begins to melt, which introduces impurities into theplasma flow. This problem is overcome by the use of the specializedmulti-cathode assembly disclosed in U.S. Pat. No. 7,589,473,incorporated herein by reference for all purposes.

FIG. 14A shows an alternative embodiment of plasma-generating device 32specifically adapted for the generation of a volumetrically oscillatingintermittent plasma flow. Such a device uses specialized multi-cathodeassembly 90 comprising multiple cathodes 91, 92, 93, held together bycathode holder 94, shown in FIG. 14B. When specialized multi-cathodeassembly 90 is used to generate intermittent plasma flows, the area ofelectric arc attachment on cathode assembly 90 settles on a singlecathode during each ‘on’ state. This allows the other cathodes to coolwhen they are not being used to pass current. Because only a limitedportion of cathodes 91, 92, 93 heat up to be able to emit electrodes,the cathode assembly will maintain a stable temperature and the area ofthe electric arc attachment will not reach cathode holder 94. While thisspecialized multi-cathode assembly is particularly suited for generatingintermittent plasma flows, it can also be used for generatingnon-intermittent plasma flows.

With reference to FIG. 8, when the plasma-generating device generates anintermittent plasma flow, the operational current level may not beachieved immediately. With reference to FIG. 15B, the operationalcurrent occurring between times t₈ and t₉ is made possible by applyingthe voltage pattern shown in FIG. 15A and passing the current shown inFIG. 15B, up to time t₈, through the cathode assembly 90 and anode 42(not shown in FIG. 14A). At time t₈, the operational current is reached,and the plasma behaves like a continuous plasma flow from time t₈ untiltime t₉ as the operational current is maintained at a constant level.During operation of such a device, the plasma flow completely ceasesduring the ‘off’ state.

During a single ‘on’ state of the intermittent plasma flow, many periodsof volumetric oscillation may occur. In one embodiment, the duration ofan ‘on’ state may be controlled through the console. For someapplications, the optimal duration of the ‘on’ state is approximately 5ms. During an ‘on’ state with this duration, a number of radial plasmaoscillations may occur. For a longer ‘on’ state, such as 1 s, there isenough time to generate a number of axial oscillations as well as radialoscillations.

By replacing the constant operational current during the ‘on’ state witha biased pulse wave such as current 50 shown FIG. 11, the intermittentplasma flow embodiment generates a volumetrically oscillating plasmaflow during the ‘on’ state. FIG. 15C shows a single period of a currentwave with these properties, which is suitable for generating avolumetrically oscillating intermittent plasma flow. In a preferredembodiment, the length of the ‘on’ state is 1-100 ms with a constantbias current level of 1-3 A. The pulse current level is preferably about30-50 A. Oscillations during the ‘on’ state are accomplished by a highfrequency biased pulse wave with a duty cycle of 0.5 and a frequency of20,000-100,000 Hz.

Referring again to FIG. 6, console 31 also contains control circuitry,such as a processor, to regulate the flow of plasma-generating gas toplasma-generating device 32. In a preferred embodiment, console 31 ismounted on service module 34 that houses the plasma-generating gassupply and preferably coolant. Alternatively, console 31 may be mountedon an overhead arm or on a cart. Console 31 preferably provides a stableand low variable flow rate of plasma-generating gas during operation. Inembodiments adapted for surgical procedures, the flow rate is keptrelatively low to ensure both a laminar plasma flow and suitable powerlevel for surgery. In a preferred embodiment, the flow rate ofplasma-generating gas during operation is about 0.1-0.6 L/min at roomtemperature, preferably about 0.2-0.5 L/min at room temperature. Theplasma-generating gas flows to device 32 through connector 33. Tomaintain a stable and constant plasma flow rate, the control circuitrypreferably maintains the plasma flow at a given flow rate with anaccuracy of about 10 mL/min. In a preferred embodiment, the operator isable to select a flow rate using console 31 starting at 0.1 L/min andincreasing in increments of about 30 mL/min.

During operation, the temperature of device 41 components, such ascathode 43, intermediate electrodes 46′, 46″, 46″, and anode 42, shouldbe kept below the smelting point. To cool these elements, in someembodiments, one or more cooling channels 40 are arranged so that acoolant circulates within plasma-generating device 41. Preferably, wateris used as the coolant, however other types of fluids may be used. In apreferred embodiment, the coolant flows in channels 40A and 40B. In FIG.7, for example, the coolant from console 31 enters device 41 throughchannel 40A, then goes around anode 42, and leaves device 41 throughchannel 40B. As the coolant traverses channels 40A and 40B, it absorbssome of the heat generated during operation of device 41.

In other embodiments, shown in FIG. 16, cooling channels pass throughdevice 41. In these embodiments, the coolant may be discharged from thedevice at coolant outlets 400 located near outlet 3. Such a coolingsystem is disclosed in U.S. Pat. Pub. No. 2007/0029292, incorporatedherein by reference for the purposes of disclosing a pass-throughcooling system. Pass-through cooling systems have some advantages.Because there is no need to return the heated coolant and all coolingchannels carry the coolant only one way, a lower coolant rate isrequired and the plasma-generating device can be miniaturized. Further,by discharging the coolant along with the plasma flow it is possible tolocalize the heating effect of the plasma flow.

Preferably, console 31 also contains control circuitry to regulate theflow of coolant through the system. By maintaining a steady flow ofcoolant through plasma-generating device 32, console 31 preventsoverheating, which can cause damage to device 32. Preferably, for theclosed cooling systems shown in FIG. 7, console 31 monitors thetemperature increase of the coolant after passing throughplasma-generating device 32. This temperature difference can be used tocalculate the power output of the device, which can be shown on thedisplay of console 31. In some embodiments, if the change in temperatureof the coolant exceeds a certain threshold, for example 10 K, thecoolant control circuitry disables the system as a safety precaution.

Connector 33 transfers current, plasma-generating gas, and coolantbetween console 31 and plasma-generating device 32. Separate connectionsfor each may be used, however, in a medical setting having multipleconnections or wires leading to plasma-generating device 32 isinconvenient. Preferably, all the required connections are enclosed intoa single connector. In one embodiment, connector 33 comprises shieldedwires for applying current to the plasma-generating device, and flexiblehoses suitable for transporting the plasma-generating gas toplasma-generating device 32 and circulating the coolant into and out ofplasma-generating device 32.

Additional components of a system for generating volumetricallyoscillating plasma flows may include a suction module for removingextraneous tissue during surgical procedures. A suction channel may beincorporated into plasma-generating device 32 that allows the suctionsystem to remove the extraneous matter from surgical site throughconnector 33 and into console 31 for storage and eventual disposal.

5.3 Generation of Volumetrically Oscillating Plasma Flow

Additional aspects of this disclosure are illustrated with reference toFIGS. 17A-C. FIG. 17A shows a high frequency biased pulse wave. The biascurrent level is shown as numeral 144. The pulse current level is shownas numeral 145. The high frequency period is shown as numeral 143. Thehigh intensity interval is shown as numeral 141 and low intensityinterval is shown as numeral 142. FIG. 17B shows a low frequency biasedpulse wave. All features of this wave are shown as the same numerals asin FIG. 17A. FIG. 17C shows a modulated biased pulse wave. All featuresof this wave are referenced by the same numerals, except the highfrequency period is shown as numeral 146 and the low frequency period isshown as numeral 143.

Turning to the physical mechanisms giving rise to the formation ofvolumetrically oscillating plasma flows, in one embodiment anoscillating current such as current 50 shown in FIG. 11 is applied to aplasma-generating gas. Plasma-generating gas heated by bias currentI_(L) forms a low intensity plasma flow, while the plasma-generating gasheated by pulse current I_(H) forms a high intensity plasma flow. Viewedover time, the resulting plasma flow is a low intensity plasma flow withbursts of high intensity plasma flow. The frequency of the highintensity plasma bursts corresponds to the current pulse frequency.

The periodic change in intensity of the plasma flow results in theoscillation of several key properties of the plasma flow, such as thedynamic pressure P_(d)(T), the static pressure p(T), the density ρ(T),the enthalpy h(T), and the power P(T), all of which are functions of theplasma temperature. The effect of these oscillations on the volume ofthe plasma flow depends upon the timescale of these oscillations. Highfrequency oscillations, i.e. oscillations with a frequency greater than2,000 Hz, result in a radially oscillating plasma flow, i.e., avolumetrically oscillating plasma flow with radial oscillations andsmall-scale axial oscillations. Lower frequency oscillations in therange of 20-100 Hz, on the other hand, result in an axially oscillatingthe plasma flow, i.e., a volumetrically oscillating plasma flow withpredominantly large-scale axial oscillations.

FIGS. 18A-C illustrate high and low frequency volumetric oscillations ofplasma flow. FIG. 18A shows active plasma 61 during the low intensityinterval. This illustration applies equally to the low intensityinterval of either low or high frequency oscillations. The volume ofactive plasma 61 of the low intensity plasma flow is relatively limited.In a preferred embodiment a bias current level I_(L) of 6 A producesplasma with a temperature of about 11,000-12,000 K. Under theseconditions, the volume of active plasma is relatively small and islimited to the proximal region 6, as shown in FIG. 1A. Outside activeplasma region 61 shown in FIG. 18A, air mixes with the plasma particlesand cools them to a temperature below 10,000 K.

FIG. 18B shows the dynamics of high frequency oscillations of a plasmaflow. The oscillation frequency is greater than 2,000 Hz, and ispreferably in the ultrasonic range, i.e. greater than 20,000 Hz. In thisexample, the duty cycle of the current wave is about 0.05-0.15. In apreferred embodiment where the pulse current level I_(H) is 30 A, theoutlet temperature of the high intensity plasma is 20,000-30,000 K.During the high intensity interval, active plasma occupies region 62.The increase in active plasma volume from region 61 to region 62 is duemostly to a significant increase in the width of the plasma flow. Thisincrease in width is accompanied by a modest increase in the length ofthe plasma flow. Significantly, when oscillations occur at a highfrequency, the high intensity plasma flow is shorter and wider than acontinuous plasma flow with an outlet temperature of 20,000-30,000 wouldbe. Due to limitations of presently known materials, however, generationof such a continuous plasma flow is presently not difficult. Such acontinuous plasma flow would have a large volume due to the presence ofa long distal region containing active plasma. In the case of highfrequency oscillations, no such distal region forms.

In contrast, low frequency current waves produce plasma flows thatexhibit significant predominantly axial oscillations. FIG. 18C shows thedynamics of a volumetrically oscillating plasma flow oscillating at alow frequency. In a preferred embodiment, the current is a biased pulsecurrent wave where the frequency of current pulses is 20-100 Hz, thecurrent wave duty cycle is 0.05-0.15, and the bias current level I_(L)and pulse current level I_(H) are 6 A and 30 A, respectively. During alow frequency oscillation, the volume of the plasma flow increases fromregion 61 to region 62, shown in FIG. 18B, and then to region 63. Thegreatest increase in the plasma flow volume is attributable to asubstantial increase in the plasma flow length. This length canoscillate as much as 40 mm, from a length of approximately 15 mm to 55mm. In contrast, after transition time t_(transition) the width of thehigh intensity plasma flow in the proximal region is only slightlylarger compared to the low intensity flow width. While the shape of thehigh intensity plasma flow resembles the shape of a continuous plasmaflow with a similar temperature, the two shapes are not the same. Thewidth of the axially oscillating plasma flow at a point significantlyremoved from the outlet along axis 4 is substantially larger than thewidth of a continuous plasma flow at the same distance. For an axiallyoscillating plasma flow discharged from a device with an outlet diameterof 0.5 mm, the width can reach up to 5-6 mm at a distance of 20-50 mmfrom outlet 3. To achieve the same width with a continuous plasma flowat 20,000-30,000 K, the outlet diameter would need to be 1.2 mm.

FIGS. 19A-C highlight the differences in volume between a continuousplasma flow and the high intensity plasma of volumetrically oscillatingflows with low and high frequency. FIG. 19A shows the volume of acontinuous plasma flow with an outlet temperature of 20,000 K dischargedfrom an outlet diameter of 0.5 mm. FIG. 19B shows a high intensityplasma flow during the high intensity interval of a high frequencyoscillation. FIG. 19C shows a high intensity plasma flow during the highintensity interval of a low frequency oscillation. In the continuousplasma flow shown in FIG. 19A, the outlet temperature is 20,000 K.Comparing FIG. 19A to FIG. 19B shows that the width of the plasma flownear the outlet is significantly greater when using high frequencyvolumetric oscillations, while the length of the plasma flow issignificantly smaller than the continuous plasma flow. Comparing FIG.19A to FIG. 19C shows that while the lengths of the two plasma flows arecomparable, the width of the volumetrically oscillating plasma flow atdistance 100 is significantly larger than the width of the continuousplasma flow at the same distance. These differences betweenvolumetrically oscillating plasma flows and continuous plasma flowsoccur because of the dynamics of the particles forming the plasma flowduring a period of oscillation.

The dynamics of the particles forming the plasma flow can be explainedas follows. The mass flow rate of the plasma, which is constant for agiven plasma-generating gas flow rate, can be expressed by the followingequation:{dot over (m)}=ρuA,where {dot over (m)} is the mass flow rate in kg/s, ρ is the density inkg/m³, u is the flow velocity in m/s, and A is the mass flow area in m².The mass flow area is a constant at outlet 3. Because the mass flow rateat this point is the same during the high intensity interval and the lowintensity interval the following relationship holds:ρ_(low) u _(low)=ρ_(high) u _(high).

Further, because the density of the plasma decreases significantly athigher temperatures:ρ_(low)>>ρ_(high),the velocity of the plasma during the high intensity portion of theoscillation is significantly larger, i.e.:u _(low) <<u _(high).If the high intensity interval t_(H) is very short, the plasma flow isdominated by the scattering behavior observed during the transition timet_(transition). This situation is illustrated in FIG. 20A. Here, thehigh intensity bursts are terminated around transition timet_(transition), so the length of the plasma flow does not have anopportunity to expand significantly to the length shown in FIG. 19C.Instead, the oscillation of the volume of the plasma flow is mostly dueto the change in width occurring as particles are scattered away fromthe plasma flow axis. Frequencies greater than 2,000 Hz produce radiallyoscillating plasma flows such as shown in FIG. 6B.

When longer pulse lengths are used, high intensity interval t_(H) can bemuch greater than transition time t_(transition). This situation occurswhen the high intensity bursts are 0.5-5 ms, which corresponds to afrequency range of 20-100 Hz, having a duty cycle of 0.05-0.15. Thissituation is shown in FIG. 20B. Here, the high intensity burst lastslong enough for the relatively high velocity, low density plasmaparticles to extend the active plasma flow to a significant length andachieve the form shown in FIG. 19C. While it appears that the radialscattering which occurs during the transition time may only effect theshape of the plasma flow at the very beginning of a long pulse, inreality the radial scattering was found to have an effect well beyondtransition time t_(transition).

Beyond just increasing the width of the plasma flow, the radialscattering of plasma particles changes the dynamic pressure of theplasma flow. Dynamic pressure can be expressed with the followingequation:

$P_{d} = {\frac{1}{2}\rho\; v^{2}}$where P_(d) is the dynamic pressure in pascals, ρ is the fluid densityin kg/m³, and v is the fluid velocity in m/s. The dynamic pressure is aproperty of plasma flows which influences how far into a medium, such asair or tissue, the plasma flow penetrates. For a plasma flow, thedynamic pressure P_(d) has the axial component, P_(da), and the radialcomponent, P_(dr). In continuous laminar plasma flows, the velocity ofthe particles is substantially aligned with the plasma flow axis, so theaxial component of the dynamic pressure, P_(da), is relatively high,while the radial component, P_(dr), is negligible. Dynamic pressurecomponents for a continuous plasma flow are reflected in FIG. 21A-C.FIG. 21A shows a constant current that generates a continuous plasmaflow. FIG. 21B shows an almost non-existent radial component of thedynamic pressure, P_(dr). FIG. 21C shows a substantial axial componentof the dynamic pressure, P_(da). The distribution of the radial andaxial component of the plasma flow dynamic pressure changes when theplasma flow oscillates volumetrically. These changes are reflected inFIG. 21D-F. FIG. 21D shows an arbitrary current pulse, that is longerthan the transition time. FIG. 21E shows the radial component of thedynamic pressure, P_(dr). In the beginning of the high intensityinterval, due to scattering, the radial component of the dynamicpressure P_(dr) quickly reaches its maximum. As the slow-moving, highdensity particles are pushed away by the fast moving, low densityparticles, the radial component P_(dr) gradually drops. At the end ofthe high intensity interval, due to the pressure gap, described inconnection with FIG. 4F, the radial component of the dynamic pressure isnegative. As the slow-moving, high density particles of the next lowintensity interval fill the gap, the radial component of the dynamicpressure P_(dr) levels off. FIG. 21F shows the corresponding axialcomponent of the dynamic pressure, P_(da). The sum of the two dynamicpressure components during the high intensity interval is constant.Accordingly, as the radial component decreases, the axial componentdecreases.

A significant radial dynamic pressure enables the volumetricallyoscillating plasma flow to penetrate into a medium substantiallyadjacent to the plasma flow, rather than only the medium transverse tothe plasma flow axis. In one embodiment, a radially oscillating plasmaflow is used to cut tissue during surgery. While the plasma flowaccomplishes efficient cutting along the plasma flow axis, the radialcomponent of the dynamic pressure enables a simultaneous penetrationinto the walls of the cut to achieve haemostasis.

The radial component of the dynamic pressure is also responsible for theincreased width of an axially oscillating plasma flow, as can be seen inFIGS. 22A-D. FIG. 22A shows low intensity plasma flow 120, right beforethe start of a high intensity interval. FIGS. 22B-D show the developmentof the high intensity plasma flow. FIG. 22B shows the radial scatteringof plasma particles, which rapidly increases the radial component of thedynamic pressure. This rapid increase of the radial component of thedynamic pressure causes a pressure wave 121, which disturbs the flow ofair surrounding the plasma flow. FIG. 22C shows that shortly after thetransition time has passed, the volume of the plasma flow begins toincrease in the axial direction. Air eddies 122 form as a result of theradial pressure wave in the beginning of the high intensity interval.Air eddies 122 create a lasting air disturbance, which continues wellpast the transition time. FIG. 22D shows the resulting expansion ofwidth 123 of the flow that occurs due to air eddies 122 interacting withthe plasma flow throughout the duration of the high intensity pulse.

Due to changes in dynamic pressure, oscillations of the plasma flowcreate acoustic waves. Specifically, expansion of the plasma flow withthe accompanying increase in dynamic pressure causes the displacement ofair molecules away from the plasma flow. On the other hand, contractionof the plasma flow, with the accompanying decrease in dynamic pressure,creates an area of low pressure around the plasma flow. Air, which is atatmospheric pressure, rushes into the area of low pressure. In a singleoscillation, air is pushed away by the expanding plasma and then suckedback into the area of low pressure, which results in a single airoscillation. The air oscillations follow the plasma flow oscillations.As the plasma flow oscillates radially and axially, the air oscillatesboth radially and axially as well. This is shown in FIG. 18B for aradially oscillating plasma flow and in FIG. 18C for an axiallyoscillating plasma flow.

When the frequency of plasma oscillations is greater than 20,000 Hz, theacoustic waves are ultrasonic, and can transmit ultrasonic mechanicalenergy from the plasma flow to a tissue or other medium. In medicine,ultrasonic waves are known to cause cavitation, which has been shown tocause or enhance coagulation. The amount of energy transferred byacoustic waves to a medium depends on the amplitude of the plasma flowvolumetric oscillations generating the acoustic waves. The amplitude ofvolumetric oscillations in a plasma flow, in turn, depends on (1) thedifference in temperatures between the low intensity plasma and the highintensity plasma, (2) the high intensity interval t_(H), and (3) thepulse frequency.

FIG. 23 shows the length of the high intensity plasma as a function ofthe period τ of oscillation. The plasma-generating device used in theexperiment had an outlet diameter of 0.5 mm and generated a biased pulsecurrent wave with a duty cycle of 0.10. To show the influence oftemperature on the length of the plasma flow, several measurements wereperformed each with a different outlet temperature of high intensityplasma. In each measurement the temperature of the low intensity plasmaflow was 10,000 K, and the length of the plasma during the low intensityinterval was approximately 8 mm, as indicated by line 131. As seen fromFIG. 23, higher outlet temperatures of the high intensity plasma resultin longer flows. Similarly, longer periods result in longer flowsbecause the plasma flow has a greater opportunity to extend.

FIG. 23 also shows how the amplitude of the volumetric oscillations varywith frequency. For high frequency oscillations, i.e., when τ is lessthan 0.5 ms (f>2,000 Hz), the length oscillations are small. In thismode, scattering of the plasma flow during the transition time is thedominant mechanism of volumetric oscillations and therefore thevolumetric oscillations are predominantly radial. For periods longerthan 5 ms significant length oscillations are observed. For example, anaxial extension of up to 50 mm for periods of 50 ms (f=20 Hz) ispossible.

Regardless of the frequency of oscillations, the difference intemperatures between the high intensity plasma and the low intensityplasma has a dramatic influence on the oscillation amplitude. Forexample, a temperature difference of approximately 5,300 K produces anoscillation with an amplitude of less than 10 mm for a period of 50 msas shown in FIG. 23, while a temperature difference of 10,000 K producesan oscillation of approximately 40 mm for a 50 ms period. Therefore, inthe preferred embodiment, the temperature of the high intensity plasmais preferably at least 10,000 K greater than the temperature of the lowintensity plasma.

It is also possible to combine low frequency axial oscillations withhigh frequency radial oscillations, so that an axially oscillatingplasma flow generates ultrasonic acoustic waves. As discussed above,applying the low frequency biased pulse wave shown in FIG. 11 results inaxial oscillations, but does not produce oscillations in dynamicpressure faster than 100 Hz. Using a high frequency biased pulse waveform produces a plasma flow that does not have significant axialoscillations. The modulated biased pulse wave shown in FIG. 12, however,can in fact produce a plasma flow that oscillates significantly inlength while still providing ultrasonic energy during the high intensityportion. Basically, the modulated biased pulse wave is a high frequencybiased pulse wave modulated by low frequency pulses. The envelope ofcurrent 150, shown in FIG. 12, resembles a current suitable for a lowfrequency axially oscillating plasma flow. In one embodiment, current150 has low frequency period τ₁ of 30 ms (f≈33 Hz) and a low frequencyduty cycle D₁ of 0.13. During the high intensity interval of thehowever, current 150 preferably modulates the high frequency pulseshaving a high frequency period τ₂ of 40 μs and a high frequency dutycycle D₂ of 0.5. High frequency duty cycle D₂ of the modulated biasedpulse wave is greater than the 0.05-0.15 used for a radially oscillatingplasma flow. The larger duty cycle does not disrupt the axial expansionduring the high intensity interval. In this preferred embodiment, theplasma flow provides both axial oscillations and radial oscillationsduring the high intensity interval. For applications that requireultrasonic energy, the frequency of the radial oscillations is in theultrasonic range, above 20,000 Hz.

5.4 Surgical Applications

Turning now to specific applications of volumetrically oscillatingplasma flows, due to the unique properties discussed above,volumetrically oscillating plasma flows are useful, for example, foraccomplishing basic surgical tasks such as cutting, vaporization, andcoagulation.

5.4.1 Coagulation

5.4.1.1 Principles of Plasma Coagulation

To appreciate the benefits of volumetrically oscillating plasma flowswhen used for coagulation, an overview of coagulation with continuousplasma flows is provided. Referring to FIG. 24, surgical site 161 iscreated by the dissection of tissue 163 during surgery. Alternatively,such a site may be created unintentionally as a result of a wound.Bleeding occurs at tissue surface 169, forming pooled blood 167. Bloodflows 165 continue at a substantially constant rate until the tissue iscoagulated. Typically, blood flow is measured in mL/min or L/min,however, it is also possible to express the bleeding rate R_(blood) inmm/s. In other words, the bleeding rate in a tissue can be measured byhow far a particle of blood travels in a unit of time.

The bleeding rate varies by tissue type, and ranges from negligible intissues such as cartilage to very intense for highly vascular tissuesuch as liver or kidney. For the purposes of this disclosure, bleedingtypes are defined as shown in table 1.

TABLE 1 Bleeding Type Bleeding Rate, R_(blood) [mm/s] Low <0.3 Medium0.3-1.0 High 1.0-2.0 Intensive 2.0-3.0 Very Intensive >3.0

For a given tissue, the bleeding rate in mm/s is typically measured asfollows. A small wound having an area A is made on the surface of anorgan. Typically, this wound is an 8-mm diameter circle. Blood iscollected from the wound over a 30-second period and the mass of theblood M in grams is measured. The bleeding rate R_(blood) is calculatedusing the following equation:

${R_{blood} = \frac{M}{30 \times \rho_{blood} \times A}},$where M is the mass of blood in grams collected in 30 seconds, ρ_(blood)is the density of blood in g/mm³, and A is the wound area in mm². Ingeneral, and especially for procedures involving high, intensive, orvery intensive bleeding rates, achieving hemostasis of bleeding tissuesis an essential surgical task.

One way of achieving hemostasis is by applying heat to the bleedingtissue. This heat creates thermal changes that result in the formationof a sealing layer, which prevents further blood flow. FIG. 25 shows asurgical site in which blood flow 165 has been stopped by forming asealing layer covering underlying tissue 163. Sealing layer 171 iscomposed of two layers: spongy layer 173 and compact layer 175. Spongylayer 173 is a region in which all fluids have been vaporized, leavingonly a solid component. The fluid component of tissue cells, blood, andinterstitial fluid make up approximately 80% of the tissue. As a result,once the fluid component is vaporized, what remains is a substantiallyporous layer, referred to as the spongy layer because it resembles asponge. Pore diameters are not uniform within spongy layer 173, butaverage pore sizes and porosities are known for particular tissue types.Table 2 presents average pore diameters and porosities for typicaltissue types.

TABLE 2 Tissue Type Diameter, d [mm] Porosity, P [%] Lung 0.06-0.0990-95 Spleen 0.04-0.07 85-90 Liver 0.035-0.06  75-80 Kidney 0.02-0.0465-70

Referring again to FIG. 25, compact layer 175 lies below spongy layer173, i.e. it is between the spongy layer and underlying tissue 163. Forconvenience, the term “below” when referring to tissue layers meansfurther away from the surface and the term “above” means closer to thetissue surface. Compact layer 175 is composed primarily of denaturedproteins present in tissue and blood. When formed, compact layer 175 isa gel-like solid and is substantially impermeable to fluid flow, therebypreventing the passage of blood flow 165 from underlying tissue 163 tothe tissue surface 169.

A preferred sealing layer has both a spongy layer and a compact layer.To completely stop bleeding in the surgical site, sealing layer 171 mustbe thick enough to withstand the pressure of blood flow 165. On theother hand, sealing layer 171 should be as thin as possible while stillachieving coagulation, because healthy tissue 163 is destroyed in theprocess of creating sealing layer 171. Minimal destruction of healthytissue is particularly important in sensitive tissues such as the brainand pancreas. In general, minimal destruction of tissue leads to reducedrecovery times.

When using a plasma flow for coagulation, the plasma flow is directed atsurgical site 161. High energy plasma particles transfer heat to thetissue by colliding with tissue molecules. Referring again to FIG. 25,heat from plasma flow 177 evaporates fluid in the tissue, which formsspongy layer 173. The plasma flow has lost heat as it passed throughspongy layer and is unable to vaporize the blood below the spongy layer.In that region, plasma flow 177 raises the temperature of the tissuehigh enough to denature protein and to form compact layer 175. Thethicknesses of the spongy and compact layers depends upon the rate atwhich plasma flow 177 transfers heat to the tissue at a given depth, andthe time during which flow 177 is directed at tissue 163.

The rate at which heat is transferred to the tissue is given by the heatflux q, measured in W/m². The heat flux can be related to thetemperature of the plasma flow in the following way:

${q = {\frac{P}{A} = \frac{{E(T)} \times F_{Plasma}}{A}}},$where power P is measured in W, enthalpy E, which is a function oftemperature, is measured in J/kg, area A measured in m², and mass flowrate F_(Plasma) is measured in kg/s. The initial power level of theplasma flow can be calculated using the temperature and mass flow rateof plasma discharged from the plasma-generating device outlet 3. As theplasma flow propagates along the flow axis, however, interactions withthe surrounding medium reduce both the temperature and the flow rate andincrease the area over which the plasma flow is distributed.

FIG. 26A shows a continuous plasma flow forming a sealing layer 171 atsurgical site 161, while FIG. 26B shows the heat flux q as a function ofdistance from tissue surface 169. As discussed above, spongy layer 173is formed when the fluid present in the tissue is vaporized. Tissuesurface 169 experiences maximum heat flux 181, and fluids there arequickly vaporized. As plasma flow 177 passes further into the tissue,transferring heat along the way, it loses energy and the heat fluxdecreases. Formation of the spongy layer 173 at a given depth occursonly if the heat flux is large enough to continuously evaporate bloodflow 165. For example, a heat flux of approximately 2.3 W/m² is requiredto evaporate a blood flow rate of 1 mm/s.

For continuous plasma flow applications, fluid boundary 179 marks thelocation where heat flux 32 is equal to the heat flux required toevaporate blood flow 165, at fluid boundary 179 liquid blood exists atits boiling point, approximately 100° C.

In compact layer 175, the heat flux is too low to evaporate the incomingblood. Consequently, blood in this region continues to flow toward fluidboundary 179. While not sufficient to vaporize blood and other fluids,the heat flux in the compact layer does raise the temperature of thetissue and blood. When a tissue is heated, protein present in tissuecells and blood undergoes an irreversible reaction, called denaturation.While denaturation of protein occurs at 40-41° C., at such temperaturesthe denaturation process takes a few hours. If a tissue is heated to 65°C., protein in tissue and blood cells denatures in time suitable forsurgery, in under 1 ms. For purposes of this disclosure, the compactlayer refers to the layer directly below the spongy layer in which asubstantial amount of protein has denatured. For a continuous plasmaflow, as shown in FIG. 26A, the compact layer is shown to extend fromfluid boundary 179 to denaturation boundary 172. In layer 163 below thecompact layer, a substantial amount of protein in the tissue is notdenatured. It should be noted that denatured protein becomes visible inmorphological samples only several hours after the tissue has beenheated, however, denatured protein prevents the flow of bloodimmediately. Accordingly, morphological samples taken within a few hoursafter the procedure may not show the true extent of denaturation.

Coagulation with prior art continuous plasma-generating devices requireda careful tuning of the plasma flow's heat flux so that the plasma flowwould create a spongy layer and a compact layer of sufficient thickness.This was typically done by precisely controlling the distance from theplasma-generating device outlet to the tissue surface for a given plasmaflow. With a continuous plasma flow, a 200-300 μm spongy layer coupledwith a 300-500 μm compact layer were reliably achieved. Thesethicknesses can be formed in tissue with low or medium bleeding levels,but cannot be formed in tissue with higher bleeding rates. Further,there are no known reliable ways to increase these thicknesses to stopheavier bleedings. Intuitively, if a higher intensity plasma flow issupplied to a tissue with a high bleeding rate, that flow sublimates thesurface of the spongy layer. This sublimation happens faster than theformation of the spongy and compact layers.

Coagulation of a tissue with a high bleeding rate with a continuousplasma flow is schematically shown in FIGS. 27A-C, which depict threeconsecutive times. In FIG. 27A, a plasma flow with a relatively highheat flux has just been directed at the tissue. Rapidly, the plasma flowforms a relatively thick spongy layer 173. Because the fluidvaporization process, which forms the spongy layer, is much faster thanthe heat diffusion process, which forms the compact layer, compact layer175 is still relatively thin. (The vaporization process refers to (1)vaporization of the fluid component or (2) simultaneous sublimation ofthe solid component and vaporization of the fluid component containedtherein. The vaporization process should not be confused with thesurgical task of vaporization discussed below.) FIG. 27B shows thetissue a brief time later, after continued application of the plasmaflow with a very high heat flux. Due to the large amount of heattransferred to the tissue at the surface, thickness 191 of the spongylayer has been completely sublimated. Despite the inward movement offluid boundary 179, spongy layer 173 has not increased in thickness.Additionally, compact layer 175 remains relatively thin, as not enoughtime has passed to denature protein in the underlying tissue. FIG. 27Cshows the tissue at yet a later time. Similarly, the result of continuedapplication of the plasma flow is that a thickness 191 of tissue hasbeen sublimated without any increase in thickness of the sealing layer.Increasing the heat flux by increasing the plasma flow temperature orshortening the distance to the tissue (which would increase the heatflux) would not be useful either. If the plasma flow has too high a heatflux, the surface of the sealing layer translates inward as tissue isvaporized without increasing the thickness of the sealing layer. Thisresult is undesirable as healthy tissue is destroyed unnecessarily andthe resulting sealing layer, having only a thin compact layer, does notform to have the required thickness to stop heavy bleedings.

The fundamental problem with continuous plasma flow coagulation isillustrated in FIG. 28. The minimum thickness of the spongy layerrequired for coagulation is at least 200 μm for a continuous plasma.FIG. 28 shows plots of plasma flow heat flux as a function of distancefrom the surface in the spongy layer. Each curve corresponding to aplasma flow with a different tissue surface heat flux. It is known thata heat flux of 5.3 W/mm² or above results in rapid tissue sublimation.It is also known that a heat flux of 2.3 W/mm² is required to vaporizeblood flowing at a rate of 1 mm/s. As seen from the graph a heat flux of4.2 W/mm² is required at the surface to establish a heat flux of 2.3W/mm² 200 μm below the tissue surface. As another example, a heat fluxof 4.6 W/mm² is required to vaporize blood flowing at a rate of 2 mm/sat a distance of 200 μm from tissue surface. As seen in FIG. 28, toachieve such a heat flux at 200 μm from tissue surface, the surface heatflux must be at least 7.4 W/mm², which exceeds the sublimationthreshold. This means that it is not possible to coagulate tissuebleeding with the blood flow rate exceeding 1.4 mm/s, which correspondsto a heat flux at the tissue surface equal to the sublimation threshold.Increasing the heat flux to cope with a high blood flow rate is noteffective because sublimation of the surface tissue would offset theformation of the spongy layer.

5.4.1.2 Coagulation with Volumetrically Oscillating Plasma Flow

By using a volumetrically oscillating plasma flow for coagulation, inparticular an axially oscillating plasma flow, it is possible to avoidthe inherent problems of continuous plasma flows. As discussed above, ahigh heat flux at the fluid boundary is required for high bleedingrates, and, as a consequence, an even higher heat flux is present at thesurface of the tissue. This surface heat flux q_(surface) causes rapidsublimation if maintained continuously, or for a prolonged time period.If, however, the heat flux is rapidly reduced after forming the spongylayer, it is possible to significantly reduce or preferably completelyeliminate sublimation. Once a certain thickness of the spongy layer isformed, even a low intensity plasma flow is sufficient to heat thetissue below the spongy layer and to form a compact layer. This lowintensity plasma has a surface heat flux q_(surface) that can bedirected at the tissue surface without causing significant sublimationbecause it is below the sublimation threshold q_(sublimation). At thesame time, the low intensity plasma has a heat flux that heats thetissue below the spongy layer, thereby forming the compact layer whilecausing only a minimal sublimation of the spongy layer.

The application of a single oscillation of low frequency volumetricallyoscillating flow is shown in FIG. 35A-B. In FIG. 29A, high intensityplasma flow 210 with corresponding length L_(High) is directed at tissue215. Because the length of a plasma flow is a function of itstemperature, the length is a useful indicator of the heat flux that theplasma flow provides. In this case, high intensity plasma flow 210 has aheat flux sufficient to rapidly form spongy layer 212 (and negligiblethickness of the compact layer 213). The high intensity interval isgiven by the preferred low frequency of 20-100 Hz and the preferred dutycycle of 0.05-0.15 of the low frequency biased pulse wave used togenerate the axially oscillating plasma flow. For example, for thefrequency of 50 Hz and the duty cycle of 0.1, the high intensity plasmais applied for 2 ms. This relatively short application of the highintensity plasma flow evaporates the blood and other fluids from theouter tissue layer. As mentioned above, continued application of highintensity plasma flow 210, however, would cause rapid sublimation atsurface 214. Furthermore, continued application of high intensity plasmamay cause the erosion of the plasma-generating device components.Accordingly, a prolonged application of high intensity plasma is neitherdesired nor feasible.

Turning to FIG. 29B, high intensity plasma flow 210 shown in FIG. 29Ahas changed to low intensity plasma flow 211 having a length L_(Low).The reduced length L_(Low) indicates that the heat flux provided by lowintensity plasma flow 211 is less than the heat flux provided by thehigh intensity plasma flow 210. This reduced heat flux does not rapidlysublimate surface 214. Low intensity plasma flow 211, however, continuesto provide heat to the tissue below the spongy layer 212 withoutexcessive destruction of spongy layer 212 forming compact layer 213.This heat increases the thickness of the compact layer, improving thestrength of the entire sealing layer. Axially oscillating plasma flows,which alternate between low intensity plasma having a relatively smalllength and bursts of high intensity plasma having a relatively largelength, exhibit a substantial improvement in tissue coagulation over acontinuous plasma flow.

FIGS. 30A-F illustrate coagulation with an axially oscillating plasmaflow over three oscillation periods. FIG. 30A shows the effect of thefirst high intensity pulse of plasma having a length L_(High). Afterthis first high intensity pulse, spongy layer 212 has a thickness ofapproximately one pore diameter. It has been experimentally determinedthat one pore diameter of spongy layer is the preferred thickness of thespongy layer that should be formed in a single burst of high intensityplasma. It should be noted that pore diameters are different fordifferent tissues, however 30 μm is a useful approximation. One porediameter is preferred for two reasons. First, this thickness is smallenough to ensure the high intensity pulse of plasma is short enough toavoid substantial sublimation of the tissue surface. Second, thisthickness is enough to form a suitably thick spongy layer of about 250μm in a reasonable number of oscillations. For example, in a tissue witha pore diameter of 30 μm, it takes approximately 8 high intensity pulsesto form a suitably thick spongy layer. Even at the lower limit of 20 Hz,the operator needs to apply the plasma to a single spot for less than0.5 s to form a suitably thick spongy layer.

FIG. 30B shows the effect of the low intensity plasma flow. Compared tothe high intensity plasma, low intensity plasma 211 has a smaller lengthL_(Low) and a correspondingly lower heat flux. As a result, during thelow intensity interval, surface 214 has not been significantlysublimated despite an extended exposure to the low intensity plasmaflow. Due to the continued application of low intensity plasma flow 211,however, protein in deeper layers of tissue continues to denature. Thisresults in an increase of the thickness of compact layer 213. The lowintensity interval determines the depth of compact layer formed in asingle oscillation of the plasma flow.

Two to four pore diameters is a preferred thickness of compact layerthat is formed in a single low intensity portion of an oscillation ofthe plasma flow. During the next burst of high intensity plasmaapproximately one pore diameter of compact layer will be converted intothe spongy layer as the plasma vaporizes the fluid component in thatregion. Forming two to four pore diameters of new compact layer during asingle low intensity interval ensures that over several oscillations thecompact layer increases in thickness at at least the same rate as thespongy layer.

The heat flux of the low intensity plasma is set to preferably have noeffect on the spongy layer and to keep the fluid boundary at the depthto which it was advanced during the high intensity interval for thetissue blood flow rate of 2 mm/s. In other words, the heat flux of thelow intensity plasma at the fluid boundary should be sufficient to keepthe fluid boundary at the same level during the low intensity interval.Of course, this cannot be always achieved. In some cases, the lowintensity plasma heat flux will exceed the heat flux required tovaporize fluids at the fluid boundary. This happens, for example, for ablood flow rate below an average blood flow rate of 2 mm/s. In thatcase, the spongy layer would slowly form during the low intensityinterval.

On the other hand, in some cases, the low intensity plasma heat flux atthe fluid boundary may be insufficient to vaporize all the incomingblood. In this case heat flux provided by low intensity plasma 211 isthat the heat flux needed to completely vaporize the incoming blood fromtissue 215, q_(f) can no longer be provided at fluid boundary 220 shownin FIG. 30A. Accordingly, the fluid boundary 220 has moved closer to thetissue surface during this low intensity plasma of the firstoscillation.

FIG. 31A shows a greater detail of the surface region of tissue 215 atthe end of the low intensity interval of the first oscillationillustrated in FIG. 30B. In FIG. 31A, layer 212 is the spongy layer andlayer 213 is the compact layer. Due to the receding of the fluidboundary 220 during the low intensity interval, however, some pores ofthe spongy layer between these two distances fill with blood and otherfluids. The blood and the fluids may enter into the spongy layer pores232 from compact layer 213 or from the deeper tissue layers throughcompact layer 213. Spongy layer 212 pores that are above the fluidboundary 220 do not fill with any fluids because the heat flux issufficient here to vaporize fluids even during the low intensityinterval.

Turning to FIG. 30C, once a suitable thickness of compact layer 213 hasbeen formed by low intensity plasma 211, a second high intensity plasmaburst 210 is directed at the tissue. As shown in FIG. 30C, spongy layer212 increases in thickness by another pore diameter due to vaporizingfluid in tissue that was previously a part of compact layer 213. As aresult, the thickness of the compact layer is reduced. This reduction ispartially offset by a slight increase in compact layer thickness as someof the protein in the tissue below the compact layer tissue denatures.

FIG. 31B shows the surface region of tissue 215 after the high intensityplasma flow in the situation depicted in FIG. 31A. FIG. 31B correspondsto FIG. 30C. In addition to advancing the fluid boundary deeper into thetissue, the high intensity plasma has evaporated the fluid component ofblood 232 (shown in FIG. 31A), which was present in the pores of spongylayer 231. Solid component 233 of the blood has been left behind, addingto the tissue-based solid component of the spongy layer. This extrasolid component increases the density of the spongy layer and improvesits sealing capability. Additionally, as discussed above, the highintensity plasma of the second oscillation sublimated approximately onepore diameter of the spongy layer.

FIG. 30D shows the low intensity interval of the plasma flow of thesecond oscillation. The effects of this low intensity plasma on thetissue are similar to the first low intensity interval described in FIG.30B. Again, blood may flow into part of the spongy layer because thefluid boundary recedes to a level closer to the surface. Even if thefluid boundary recedes, this flow of blood into the spongy layer willtypically be slower than during the low intensity interval of the firstoscillation. This is due to the significant increase in the compactlayer thickness that occurred as heat was provided to the underlyingtissue. In general, until a given spot has been fully coagulated lessblood and other fluids flow into the spongy layer during each subsequentlow intensity interval. FIGS. 30E and 30F show the high and lowintensity portions of the third oscillation in the plasma flow,respectively, which have effects on the tissue similar to the earlieroscillations.

FIG. 32 shows a completed a sealing layer formed by an axiallyoscillating plasma flow. Spongy layer 240 is composed of severalsublayers. Each sublayer was formed by a high intensity plasma burst. Incontrast, a spongy layer formed with a continuous plasma flow does nothave such a multilayer structure. For the cases in which the fluidboundary recedes during the low intensity intervals, the density of thespongy layer generally increases toward the surface. This is due to thepresence of red blood cells that remain in the spongy layer after theblood that flows into the spongy layer during the low intensityintervals evaporates, leaving additional solid components.

The end result of applying an axially oscillating plasma flow tobleeding tissue is the formation of a strong and robust sealing layerthrough the alternating process described above. In general, in a singleaxial oscillation of the plasma flow, spongy layer tissue is rapidlyformed during the high intensity interval while compact layer is formedduring the low intensity interval.

The discussion so far has focused on the coagulation of a single spot.In practice, the operator must coagulate an area of tissue greater thanthe spot diameter. To do this, the operator sweeps the surgical sitewith the plasma flow by moving the plasma-generating device parallel tothe tissue surface. FIG. 33A shows surgical site 270 covered with blood273. Blood 273 obscures the surgical site and the bleeding must bestopped before the operator can resume the surgical procedure. In apreferred embodiment, the operator sweeps across the surgical site 270along an exemplary path 271 shown in FIG. 33B. As the plasma flow passesover any particular position, several oscillations of the plasma flowoccur and form a sealing layer in the tissue as described with referenceto FIGS. 30A-F.

FIG. 34A-C show the effect of such a plasma flow as it sweeps alongexemplary path 271. Plasma flow 280 moves along tissue 281 at a rateslow enough to ensure that a sealing layer of appropriate thickness isgenerated at each position. As discussed above, spongy layer 282 ispreferably 250 μm thick and compact layer 283 is preferably at least200-500 μm thick. For a typical tissue with a pore diameter of 30 μm, a250 μM thick spongy layer requires approximately 8 high intensity pulses(each pulse forms approximately one pore diameters of spongy tissue).Each spot of tissue 281 should therefore be subjected to 8 oscillationsof the plasma flow in order to generate the appropriately thick spongylayer.

FIG. 35 shows a more realistic situation where the bleeding rate in thetissue varies over a single surgical site, indicated by the relativesize of the arrows 290 and 291 in tissue 281. Spots with lower levels ofbleeding can be coagulated with a thinner sealing layer. This ispreferable because less healthy tissue is destroyed to form the sealinglayer. In practice, the operator can sweep the plasma flow quickly oversurgical site 270 to create a thin sealing layer. This thin sealinglayer coagulates areas of low bleeding 290 but areas of high bleeding291 continue to bleed. This quick sweep may result in, for example, 5high intensity pulses per spot diameter. Once this thin sealing layer isestablished, the operator focuses on areas of high bleeding 291, wherecontinued exposure to the plasma flow creates a thicker sealing layer.This alternative method ensures that the sealing layer is of theappropriate thickness at each spot of the treated surface. This variablethickness sealing layer is shown in FIG. 35, where spongy layer 282 andcompact layer 283 are thick in areas of high bleeding 291 but are thinin areas of low bleeding 290.

The rate of sealing layer formation may vary with variations of thedistance between the plasma-generating device and the tissue. Ideally,the operator holds the plasma-generating device at a constant distancefrom the tissue as he sweeps along exemplary path 271 shown in FIG. 33B.In reality, the operator may not be able to keep the plasma-generatingdevice perfectly level throughout the procedure. Therefore, to accountfor operator hand movements, it is important that the coagulation effectdoes not change substantially with distance over a suitable range. FIG.36 shows the temperature of a plasma flow along the plasma flow axisduring both the high intensity portion and the low intensity portion ofa volumetrically oscillating plasma flow. Region 313, where thetemperatures of both the high and low intensity plasma are substantiallyconstant, is ideal for performing coagulation. This region correspondsto approximately 10-30 mm when the high intensity plasma has an outlettemperature of 20,000-30,000 K and the low intensity plasma has anoutlet temperature of about 11,000 K. During coagulation the operatormaintains this distance, within the range of 10-30 mm, preferably 15-25mm, ensuring that the coagulation effect does not vary significantlydespite unavoidable movement.

The speed that an operator can coagulate an area of bleeding tissue alsodepends on the spot diameter. FIG. 37 shows the required path 272 that aplasma flow with a relatively small spot diameter would have to travelin order to achieve coagulation in tissue 270. If the spot diameter atthe distance of 10-30 mm is relatively small, coagulation of a surgicalsite takes too long and is not feasible. For this reason, prior artcoagulation devices had large outlet diameters, in the range of 0.8-1.2mm. Such large diameters are acceptable, and may be even preferred, whenthe plasma-generating device is only intended to be used forcoagulation, but are too large to be used for cutting. An axiallyoscillating plasma flow, however, has an increased width compared to acontinuous plasma flow, as explained above with reference to FIGS.22A-D. For an axial oscillation, disturbances in the airflow surroundingthe plasma flow cause the plasma flow spot diameter to increase. Thisallows a plasma-generating device with a small outlet diameter togenerate a volumetrically oscillating plasma flow with a relativelylarge spot diameter suitable for coagulation. For example, in apreferred embodiment with a 0.5 mm diameter outlet, at the preferreddistance of 10-30 mm the spot diameter may reach 5-6 mm. To achieve thesame spot diameter with a continuous plasma flow, an outlet diameter of1.2 mm would be required.

In a preferred embodiment, when the plasma-generating device isprogrammed to operate in coagulation mode, the characteristics of anaxially oscillating plasma flow (e.g. the duty cycle, frequency, andtemperature) are optimized to achieve efficient coagulation. As shown inFIG. 20B, the plasma flow alternates between high intensity plasma andlow intensity plasma. The characteristics of the high and low intensityplasma agree with mathematical models of the coagulation processes andwere also confirmed experimentally.

Principal processes in tissue coagulation include spongy layer andcompact layer formation. Spongy layer formation can be modeled with thefollowing equation:q _(f)=(u _(f) +u _(b))×ρL,where q_(f) is the heat flux required at the fluid boundary in W/mm²,u_(f) is the velocity of the fluid boundary as it moves inward in mm/s,u_(blood) is the blood flow rate in mm/s, ρ_(tissue) is the density ofthe tissue in kg/mm³, and L is the energy required for vaporization inJ/kg. The density ρ_(tissue) and the vaporization energy L can beapproximated with the density and vaporization energy of water (10³kg/m³ and 2.26×10⁶ J/kg, respectively). The velocity u_(f) can berewritten as

${u_{f} = \frac{d}{t_{H}}},$where d is the desired thickness of the spongy layer formed in one highintensity burst in mm, and t_(H) is the high intensity interval in s. Asdiscussed above, d is preferably one pore diameter d_(P). The equationfor the heat flux required at the fluid boundary can therefore berewritten as

$q_{v} = {\left( {\frac{d_{p}}{t_{H}} + u_{b}} \right) \times \rho\;{L.}}$

FIG. 38 shows the graph of vaporization heat flux q_(f) plotted forthree different bleeding rates as a function of t_(H). As seen from FIG.38, the bleeding rate has a much stronger influence on the required heatflux for longer high intensity pulses. For example, for a high intensityinterval of 8 ms, the difference in vaporization heat flux q_(f) is 40%,when the blood flow rate changes from 1 mm/s to 3 mm/s.

To ensure coagulation of tissues with different bleeding rates, theplasma flow must provide the heat flux required to stop the highestbleeding rate. Referring back to FIG. 38, the plasma flow must provideat least 16 W/mm² to stop bleeding with a rate of 3 mm/s when t_(H) is 8ms. If such a plasma flow is directed at tissue with a lower bleedingrate of 1 mm/s, there will be excessive heating of the spongy layer,which may result in significant undesired sublimation. Because, asmentioned above, the bleeding rate may vary significantly over thesurgical site, this problem occurs as the operator is coagulatingdifferent spots on the same tissue. Keeping the high intensity intervalshort eliminates this problem. For example, if the high intensity plasmaburst is only 1 ms long, the required heat flux varies only 5% betweenbleeding rates of 1 mm/s to 3 mm/s. Therefore t_(H) is preferably keptlow. Additionally, exposing tissue to shorter high intensity bursts,such as 1 ms, avoids pyrolysis and charring because such a shortduration, while sufficient to vaporize fluids, is not sufficient for thepyrolysis reaction to begin.

FIG. 38 also shows that if t_(H) is very short, i.e. less than 1 ms, theheat flux required to form one pore diameter of spongy tissue becomesextremely high. And because q_(f) is the heat flux required at the fluidboundary, the corresponding heat flux at the surface q_(surface) is evenhigher. Accordingly the high intensity interval is preferably about 1ms.

The compact layer is formed by heat diffusion across the fluid boundaryinto the underlying tissue. Compact layer formation depends primarily onthe duration of the applied low intensity plasma. This heat diffusionprocess can be modeled by the Bio-Heat diffusion differential equation:

${{\rho\; C\frac{\partial T}{\partial t}} = {{k\;{\nabla^{2}T}} + h_{m} + h_{b}}},$where ρ is the tissue density in kg/mm³, C is the specific heat of thetissue in J/kg·K, h_(m) is the rate of metabolic heat production perunit volume of tissue and h_(b) is the rate of heat transfer betweenblood and tissue per unit volume of tissue in J/kg·s. The rate ofmetabolic heat production h_(m) is so much lower than the heat flux fromthe plasma flow that it can be ignored, and the rate of heat transferbetween blood and tissue h_(b) can be expressed by:h _(b)=ρ_(blood) C _(pb)ω(T _(a) −T),where ρ_(blood) is the density of blood (approximately 1050 kg/m³),C_(pb) is the heat capacity of blood (which is around 3,600 J/kg·K), ωis the blood perfusion (which is on the order of 1-10 s⁻¹), and T_(a) isthe temperature of the arterial blood flowing into the volume (which isapproximately 36.6° C.).

Using this equation it is possible to calculate the thickness of thecompact layer formed as a function of time. These calculations canprovide guidance on the optimal duration of the low intensity intervalof the plasma flow. For short times, the solution of the above equationyields the following analytical approximation:h=20×√{square root over (t _(L))},where h is the thickness in μm and t_(L) is the duration of the lowintensity interval of an oscillation (in s). FIG. 39 shows this shorttime approximation plotted alongside the numerical simulation of compactlayer formation as a function of t_(L). It is clear from FIG. 39 thatthere is a substantial agreement between the two curves for times lessthan 70 ms. As mentioned above, preferably two to four pore diameters ofcompact layer are generated in a single period of oscillation. At thelower limit, it takes at least 10 ms to produce a compact layer of twopore diameters (assuming 30 μm pores).

The rate of compact layer creation drops off as time increases. Thisrate is expressed as a derivative of h:

$h^{\prime} = {\frac{10}{\sqrt{t_{L}}}.}$FIG. 40 shows the rate of compact layer formation as a function oft_(L). As seen from FIG. 40, for t_(L) greater than 60 ms the rate ofcompact layer formation is less than the minimum bleeding rate intissue, which is assumed to be 1 mm/s. Accordingly, application of thelow intensity plasma flow beyond 60 ms does not form additional compactlayer. Therefore, the low intensity interval should be 10-60 ms, and ispreferably 15-35 ms.

Based on these biological considerations as well as other requirements,it is possible to determine the optimal characteristics of an axiallyoscillating plasma flow generated with a low frequency biased pulse wavecurrent. From device requirements, the preferred duty cycle D of0.05-0.15 ensures that the average current remains low while stillachieving a high peak current. The high current level I_(H) ispreferably 30 A and the preferred low current level is 6 A. Using such acurrent wave, the preferred volumetrically oscillating plasma flowoscillates between the preferred low temperature of at least 11,000 Kand the preferred high temperature of 20,000-30,000 K.

To produce plasma flows suitable for multiple surgical tasks, such ascoagulation cutting, and vaporization, the outlet diameter is preferably0.5 mm. With this preferred outlet diameter, an axially oscillatingplasma flow adapted for coagulation has a spot diameter of 5-6 mm whenthe device is held 15-25 mm from tissue. Under these circumstancescoagulation in the affected spot is accomplished in 250-400 ms. The timeof human reaction is approximately 200-300 ms. This means that theoperator does not need to focus on coagulating a given spot. Theoperator can move the device without regard to the speed with which hedoes, while accomplishing reliable coagulation.

As determined above, when examining the formation of the spongy layer,each tissue spot requires 8 high intensity plasma bursts to form theminimum thickness of the spongy layer. To ensure the delivery of 8pulses to a given spot, the frequency of at least 20 Hz, which is thepreferred lower limit for longitudinal volumetrically oscillating plasmaflows for coagulation, should be used. For a wave with a frequency of 20Hz with a duty cycle D of 0.05-0.15, the period τ of 50 ms and a highintensity pulse interval t_(H) of 2.5-7.5 ms. This is the minimumfrequency at which coagulation is reliable with an axially oscillatingplasma flow generated using a low frequency biased pulse wave current.In the preferred embodiment, the biased pulse wave current has afrequency of 40 Hz with a duty cycle D of 0.05-0.15, it has a period τof 25 ms and a high intensity pulse interval t_(H) of 1.25-3.75 ms.

An upper boundary on the frequency can be found by examining the amountof compact layer tissue formed in 8 oscillations of the plasma flow.FIG. 41 shows a plot of the compact layer thickness as a function of thenumber of oscillations for several different frequencies. The thicknessof the compact layer should be 200-500 μm. Using the minimum thicknessas a boundary condition, the frequency of plasma oscillations ispreferably less than 70 Hz. A volumetrically oscillating plasma flowgenerated by a current wave with a frequency of 70 Hz with a duty cycleD of 0.05-0.15 has a period τ of 14 ms and a high intensity intervalt_(H) of 0.7-2.1 ms. If the frequency is increased above this preferableupper boundary, the thickness of the compact layer is insufficient forcoagulation.

When coagulating tissue, severed blood vessels must also be sealed tofully stop bleeding. In some organs, vessel blood flow rate may approach30 mm/s. FIG. 42 shows blood vessel 275 having blood vessel wall 274,which exposed through the process of coagulation. These blood vesselscan be sealed by heating the inside walls of the exposed blood vessel,as disclosed in U.S. application Ser. No. 12/696,411 incorporated byreference herein for all purposes. This heating causes collagen in bloodvessel wall 274 to denature and swell inward, closing off the bloodvessel 275. For small blood vessels (less than approximately 1 mm indiameter) heating the tissue surface is sufficient to seal the bloodvessels. Sealing larger blood vessels requires heating of the bloodvessel walls to depth of 1-1.5 blood vessel diameters. Heating to thisdepth can only be accomplished using a plasma flow if the plasma flowhas a high dynamic pressure directed into the blood vessel. A continuousor axially oscillating plasma flow with an outlet temperature greaterthan 11,000 K can provide a dynamic pressure along the plasma flow axismeeting this criteria. At this temperature, the plasma has enough energyand velocity to vaporize blood, penetrate into the blood vessel to therequired depth, and heat the blood vessel walls from the inside. In apreferred embodiment, the volumetrically oscillating plasma flow, evenduring the low intensity interval, has an outlet temperature of over11,000 K, and is therefore suitable for sealing large blood vessels, asdisclosed in U.S. application Ser. No. 12/696,411.

FIGS. 43A-C show the application of an axially oscillating plasma flowto seal a blood vessel. In FIG. 43A plasma flow 320 has been directedinto an exposed blood vessel. Blood 321 has been vaporized to a depth of1-1.5 blood vessel diameters. Plasma flow 320 heats the exposed bloodvessel walls 322 with which it comes in contact. FIG. 43B shows exposedblood vessel walls 322 swelling inwards toward each other. After enoughswelling has occurred, walls 322 completely seal the blood vessel andstop blood 321 from flowing to surface. This swelling may not completelyocclude the vessel, particularly if the diameter of the vessel isrelatively large i.e., greater than 3 mm. In this case, the operatorsweeps the tissue surrounding the contracted vessel with the plasma flowin a circular motion. This heats the surrounding tissue, which alsoswells and forces the blood vessel walls to completely occlude thevessel.

5.4.1.3 Coagulation with Modulated Plasma Flows

In another embodiment, high frequency oscillations are introduced duringthe high intensity interval of an axially oscillating plasma flow. Thisvolumetrically oscillating plasma flow is generated by the modulatedbiased pulse current wave, an example of which is shown in FIG. 12. Likethe embodiment using the low frequency biased pulse wave shown in FIG.11, this plasma flow is also specially suitable for coagulation. Bymaintaining the same bias current level I_(L), pulse current levelI_(H), low frequency, and low frequency duty cycle as the low frequencybiased pulse wave, this embodiment also produces a plasma flow withaxial oscillations that efficiently accomplish coagulation, as describedabove. The high frequency oscillations, which produce radialoscillations during the high intensity interval of the plasma flow,occur with a frequency greater than 2,000 Hz, preferably greater than20,000 Hz. The duty cycle of high frequency oscillation is 0.35-0.65,preferably 0.5. For the high frequency oscillations of 2,000 Hz with aduty cycle D of 0.35-0.65 the period τ is 0.5 ms and a high intensitypulse interval t_(H) of 175-325 μs. A high intensity pulse intervalcorresponding to 20,000 Hz and D of 0.35-0.65 is correspondingly17.5-32.5 μs. The duty cycle D of 0.36-0.65 ensures that the length ofthe plasma does not decrease between high frequency bursts. In otherwords, the length of the plasma flow is kept at its expanded stateduring the high intensity interval. If the high frequency oscillationsare ultrasonic, these high frequency oscillations improve thecoagulation effect by additionally providing ultrasonic pressure wavesto the tissue.

Ultrasonic pressure waves, such as those provided by a plasma flowoperating at a high frequency, have at least two effects on tissue.These effects are observable when the frequency of oscillations isgreater than 20 kHz. First, acoustic vibrations at this frequencygenerate heat. This vibration heat is, however, negligible compared tothe heat provided by the plasma flow itself at outlet temperatures of11,000 K and above. Second, and more important for coagulation, thenature of heat transfer to the compact layer changes in the presence ofcavitation. Specifically, plasma that does not oscillate ultrasonicallyheats the compact layer by diffusing heat into the tissue from the fluidboundary. In contrast, plasma utilizing ultrasonic oscillations has anadditional cavitation mechanism for heating the tissue. Cavitationrefers to the action of the ultrasonic pressure waves on gas bubbles ina liquid. As explained above, the oscillations of the plasma flow resultin acoustic oscillations, which are pressure waves. When pressure wavesact on a liquid, bubbles of gas can form in the liquid when the pressuredrops below the characteristic vapor pressure of that liquid. When thepressure waves are ultrasonic, these bubbles oscillate violently andthen implode creating powerful localized shockwaves. These mechanismsrapidly heat the surrounding blood and tissue. Cavitation results infaster compact layer formation and improved coagulation.

FIG. 44 shows the effect of an ultrasonic pressure wave on the size of abubble. FIG. 44 is a plot of the bubble size as a function of time.Interval 331 corresponds to the formation of the bubble during a lowpressure wave front of the ultrasonic wave. The bubble expands when thepressure drops and contracts when the pressure increases. Interval 332shows the bubble contracting during the following high pressure wave.The bubble alternates between expansion and contraction, but expandsfaster than it contracts. When the ultrasonic pressure wave is generatedby high frequency oscillations of a plasma flow, the cycles of expansionand contraction follow the high frequency oscillations. The expansionand contraction of bubbles creates localized fluid flow. These localizedfluid flows can have high velocities and shear stresses great enough todestroy cells and other cellular structures in a tissue.

At high ultrasonic intensities the bubble shown in FIG. 44 will implodeafter a few oscillation cycles. During implosion, very high shearstresses, shock waves, pressures, and temperatures are produced. Theresult of these explosions is the destruction and fragmentation of thelocal structure of the tissue. Additionally, the heat generated from theshear stresses and bubble collapses can denature protein and increasethe rate of coagulation.

Because plasma is an ionized gas, there are important synergetic effectswhen ultrasonic energy is coupled with a plasma flow to achievecoagulation. As the plasma flow passes through the spongy layer of atissue it impacts the fluid boundary. Some of the plasma passes into thetissue below the spongy layer as dissolved gas. This increased amount ofdissolved gas then forms bubbles which act as receivers of ultrasonicenergy and cavitate. The result of combining a high-temperature plasmawith ultrasound is that the cavitation effect is greatly enhanced, whichimproves coagulation.

Morphological samples show that when ultrasonic oscillations are usedduring coagulation, the compact layer has two distinct sublayers. Thesublayer adjacent to the spongy layer is a dense homogeneous layer withall cell structure destroyed due to cavitation. The sublayer below isthe regular compact layer as described above. The resulting sublayer ofthe compact layer formed as a result of cavitation is a dense homogenouslayer that shows improved sealing characteristics. Also, a spongy layerformed from tissue that has been disrupted by cavitation has asignificantly reduced pore size, ranging from 20-25 μm rather than 30-70μm.

The addition of ultrasound is also beneficial for sealing bleeding bloodvessels. Cavitation occurring in the vessel walls and blood inside ofthe vessel accelerates the sealing process. Destruction andfragmentation of the vessel tissue greatly enhances the swelling processwhich forms the seal of the blood vessel. It has been observed that theaddition of ultrasonic high frequency oscillations increase the swellingof blood vessel walls by up to five times and greatly speed up thesealing process. Accordingly, even large blood vessels with a high bloodflow rate can be sealed rapidly without a need to direct the plasma flowinto the blood vessel and then at the surrounding tissue for an extendedperiod of time.

Accordingly, in this preferred embodiment, where a volumetricallyoscillating plasma flow is generated by a modulated biased pulse currentwave, the operator simply sweeps the tissue with the plasma flow withoutregard to the nature of the tissue and the presence of the bloodvessels. The improved coagulation properties for such a plasma flowfacilitate the rapid sealing of the blood vessels without spending extratime directing the plasma flow into the vessel. No special attentionneeds to be paid to blood vessels, as even large blood vessels with ablood flow rate of 30 mm/s are sealed rapidly by this sweeping process.

5.4.2 Cutting with Radially Oscillating Plasma

Beyond tissue coagulation, plasma flows can also be used to accomplishthe surgical task of cutting. During this surgical task a small regionof tissue is destroyed in order to separate the tissue. By separatingthe tissue, the operator can remove unwanted tissue or expose underlyingtissues for further surgery. For coagulation as described above, certainthermal and mechanical effects were accomplished in the tissue whileavoiding significant sublimation. When cutting using a plasma flow,however, sublimation of the tissue is intended. In the preferredembodiments sublimation is accompanied by the simultaneous coagulationof the just-separated tissue. This simultaneous coagulation isaccomplished by the use of a volumetrically oscillating plasma flow.Using the preferred embodiments, even tissues with characteristicallyhigh bleeding rates can be cut without significant bleeding or withoutany bleeding.

As an overview, prior art continuous plasma flows have been used toaccomplish cutting. A typical plasma flow suitable for cutting had asignificant axial component of the dynamic pressure and provided a highheat flux. A thin cut was achieved by using a plasma-generating devicewith a small outlet diameter. By keeping the cut thin, tissue damage waslimited to a very small region of the tissue and the precision of thecut was high. Coagulation effects, such as the formation of spongy andcompact layers, were negligible. The result of using a continuous plasmaflow for cutting was a thin, precise cut which bled significantly. Thebleeding tissue exposed by this cut would typically be coagulated usinga separate device.

FIGS. 45A-C illustrate the process of cutting with a typical prior artcontinuous plasma flow. Plasma-generating device 340 is placed at thesurface 341 of the tissue. By keeping plasma-generating device 340adjacent to the surface of the tissue, the surface of the tissue is incutting region 311 as shown in FIG. 36. Referring back to FIG. 45A,plasma flow 342 vaporizes a portion of the tissue, forming a cut. Thecut has walls 343 which are tissue exposed due to the destruction ofadjacent tissue. Because the radial component of dynamic pressure in acontinuous plasma flow is negligible, walls 343 are not coagulated bythe continuous plasma flow and begin to bleed. At the bottom of the cuta thin sealing layer consisting of spongy layer 344 and compact layer345 is formed. However, due to the very high heat flux provided by theplasma flow, this sealing layer translates inward rather than increasingin thickness.

To lengthen the cut, plasma-generating device 340 is moved along thesurface of the tissue to form a groove. To cut to a greater depth orcompletely separate the tissue into separate pieces, plasma-generatingdevice can be moved deeper into the cut to sublimate more tissue. Thisis shown in FIG. 45B. Additionally, during the cutting process bloodvessels 346 are dissected by the plasma flow and their exposed ends openinto the cut. Importantly, a continuous plasma flow used for cuttingdoes not efficiently coagulate the tissue or blood vessels exposed alongthe walls of the cut. Tissue bleeding 347 and vessel bleeding 348, shownin FIG. 45C, must be coagulated in some other way, such as a separateplasma flow adapted for coagulation or by some other means.

By using a radially oscillating plasma flow, cutting can be accomplishedwhile simultaneously coagulating tissue exposed by the cut, resulting ina cut with little to no bleeding. In the preferred embodiment, a highfrequency biased pulse wave of current is used to generate a plasma flowwith substantial radial oscillations. The frequency of the oscillationsis greater than 2,000 Hz, and is preferably 20,000-30,000 Hz. In oneembodiment, the biased pulse wave current has a frequency of 20,000 Hzwith a duty cycle D of 0.05-0.15, it has a period τ of 50 μs and a highintensity pulse interval t_(H) of 2.5-7.5 μs. An example of such aplasma flow is depicted in FIG. 20A. At such a high frequency, thescattering of plasma particles directs plasma both axially and radially.The radially directed plasma creates a substantial radial component ofthe dynamic pressure and heat flux. These substantial radial components,unique to volumetrically oscillating plasma flows, are used to coagulatethe tissues along the walls of the cut as the cut is made. Additionally,because the high intensity interval of the high frequency pulses is notlong enough for the plasma flow to extend to a significant length, theenergy of the plasma flow, even during the high intensity interval, isconcentrated in a volume near the outlet of the plasma-generatingdevice.

This concentration of energy is apparent FIGS. 18A-B. The volume ofactive plasma tapers to a point on the plasma flow axis, which can beconsidered the “cutting tip” of the plasma flow. Because of the veryfocused cutting tip, the resulting cut formed by the radiallyoscillating plasma flow will be thin and minimize the amount of tissuedestroyed. This cutting tip is present for both high and low intensityplasma.

A radially oscillating plasma flow accomplishes both the sublimation andcoagulation of tissue simultaneously. FIGS. 46A-C illustrate the cuttingprocess (both sublimation and coagulation of tissue) with a radiallyoscillating plasma flow. Turning first to the cutting aspect of theprocess, FIG. 46A shows a high intensity flow 350 is directed fromplasma-generating device 351 during the high intensity interval of thehigh frequency biased pulse current wave. Plasma-generating device 32 ispositioned at the tissue surface. The high intensity plasma, which hasan outlet temperature of 20,000-30,000 K, sublimates tissue to form acut. Because the high intensity plasma with the outlet temperature of20,000-30,000 Hz is maintained for just a few μs, it does not causeextended damage to the tissue. The high intensity plasma has a heat fluxdirected both axially and radially, so that the width of the cutcorresponds to the maximum width of the plasma flow, which is may be ashigh as 1.5 mm plasma is relatively wide of the outlet with the diameterof 0.5 mm.

FIG. 46B shows a subsequent low intensity interval of the plasma flowfollowing the high intensity pulse shown in FIG. 46A. The low intensityplasma has the outlet temperature of at least 11,000 K and a heat flux adistribution with a significant axial component and a relatively smallradial component. This ensures that during the low intensity interval,no radial sublimation of tissue occurs. The low intensity plasmaadvances the cut further in the axial direction, deepening it.Accordingly, both high intensity and low intensity plasma providecontribute the tissue cutting.

As to coagulation, as shown in FIG. 46A, the radial oscillations createa coagulation effect in all directions similar to the effect the axialoscillations have in the axial direction for tissue coagulation.Specifically, during a high intensity interval, the heat flux radialcomponent of the plasma flow creates spongy layer 353 in the walls ofthe cut. At the same time, the heat flux axial component of the plasmaflow creates spongy layer 353 along the bottom of the cut. Additionally,the high intensity plasma burst begin to forms compact layer 354 as heatdefuses past spongy layer 353. By the end of the high intensityinterval, compact layer 354 is relatively thin because the interval isshort compared to the time required for significant heat diffusion fromthe fluid boundary 355. Accordingly, only a very small thickness oftissue below the spongy layer has a substantial amount of denaturedprotein.

As shown in FIG. 46B, during the subsequent low intensity interval, lowintensity plasma 352 provides a lower heat flux to the tissue in thewalls of the cut. This lower heat flux increases the thickness ofcompact layer 354 without causing further sublimation of spongy layer353 in the walls of the cut. After a number of volumetric oscillations,the walls of the cut are completely coagulated. The result is shown inFIG. 46C, where the tissue has been completely separated while at thesame time a thick sealing layer comprising spongy layer 353 and compactlayer 354 completely prevents bleeding.

When the frequency of oscillations is ultrasonic, the coagulative effectof a volumetrically oscillating plasma flow on a tissue is improved bythe addition of ultrasonic energy. As described in reference to thesurgical task of coagulation, this ultrasonic energy acts as an extrasource of heating and causes cavitation in the tissue. Cavitation whichoccurs at the tissue surface aids the surgical task of cutting bydisrupting the tissue at the surface and causing fragmentation. Thiscavitation, in addition to the sublimation, results in an enhancedcutting efficiency.

To briefly review, in the surgical task of coagulation the addition ofultrasonic energy results in a greatly improved sealing layer. Likewise,ultrasonic energy dramatically improves coagulation during the surgicaltask of cutting. First, rapid heating of tissue below the fluid boundaryvia the cavitation mechanism speeds up the denaturation reaction thatforms the compact layer. This means that compact layer formation, whichis slow compared to spongy layer formation, is greatly sped up. Second,tissue disruption due to ultrasonic cavitation has many beneficialeffects on the strength of the sealing layer. Spongy layer porediameters, for example, are significantly decreased, which makes thislayer denser and better at stopping blood flow. Cavitation also createsa sublayer of the compact layer in which the cells themselves have beendisrupted, creating a more homogenous structure which is better atstopping blood flow. The result of these effects is that the sealinglayer can be made significantly thinner while at the same time beingsignificantly better at stopping bleeding.

With regards to blood vessels dissected during the cutting process, FIG.47 shows how the radial heat flux component of a radially oscillatingplasma flow can seal blood vessels. As discussed in the case ofcoagulation, penetration of plasma into blood vessels requires theplasma to have a low density and a high dynamic pressure directed intothe blood vessel. Because high intensity plasma 360 has a large dynamicpressure with a significant radial component, it is able to provide alarge dynamic pressure 361 into blood vessel 362. The high intensityplasma is capable of penetrating into blood vessel 361 and heatingexposed blood vessel walls 363 to create the sealing effect described inreference to FIGS. 43A-C.

Cavitational effect due to ultrasonic waves generated as a result ofradially oscillating are capable of completely occluding the severedvessel openings as the cutting continued to be performed to ultrasonicoscillations. Similar to the surrounding tissues, the blood vessel wallsand blood are disrupted and coagulated by the ultrasonic acoustic wave.The cavitational effect works in conjunction with the penetration of theplasma into the vessel to accomplish vessel sealing.

Because of the benefits associated with ultrasonic acoustic waves, inthe preferred embodiment for the surgical task of cutting the frequencyof oscillations is ultrasonic. The volumetrically oscillating plasmaflow is preferably generated using a high frequency biased pulse waveoscillating with a frequency of 20,000-30,000 Hz. In this preferredembodiment, the bias current level is 6 A, the pulse current level is 30A, and the duty cycle is 0.05-0.15.

Results in animal testing have shown that effective coagulation can becoupled with the surgical task of cutting using the preferredembodiment. Tissues with very intensity characteristic bleeding rateshave been cut without any significant, or any, bleeding. Additionally,vessels with diameters as large as 4 mm have been sealed as they werecut.

5.4.3 Vaporization with Radially Oscillating Plasma

In addition to the surgical tasks of coagulating and cutting, a plasmaflow can also be used to accomplish the surgical task of vaporization.Like cutting, the primary goal of vaporization is the destruction oftissue. In cutting, a small amount of the tissue was destroyed toseparate pieces of tissue from each other. In vaporization, on the otherhand, a certain amount of unhealthy or otherwise undesirable surfacetissue is destroyed. In such a situation, the goal is to completelydestroy the undesirable tissue while causing as little collateral damageas possible. This collateral damage includes incidental vaporization ofadjacent healthy tissue and bleeding from the tissues exposed duringvaporization. For example, cancerous tumor nodules situated on a tissuethat must be preserved such as on the spinal cord, brain, or ovaries mayneed to be vaporized rather than excised to protect the delicate tissuebeneath.

An ideal plasma flow for vaporization should provide precise control ofthe depth of tissue vaporization while at the same time efficientlycoagulating the tissue exposed during vaporization. A radiallyoscillating plasma flow can be used for this purpose. In fact, the sameradially oscillating plasma flow preferred for the surgical task ofcutting is also preferred for the task of vaporization. That is, thevolumetrically oscillating plasma flow is generated using a highfrequency biased pulse wave current having a frequency greater than2,000 Hz, preferably 20,000-30,000 Hz. The duty cycle of thevolumetrically oscillating plasma flow is preferably 0.05-0.15, the biascurrent is 6 A, and the pulse current is 30 A. A radially oscillatingplasma flow generated with this high frequency biased pulse current wavewas discussed with reference to FIG. 20A. In this case, short highintensity intervals limit the length of the plasma flow unlikeconcentrating energy close to the outlet of the plasma-generatingdevice. Additionally, as described above with reference to cutting, sucha volumetrically oscillating plasma flow is optimized to efficientlycoagulate tissue exposed after vaporization of the surface. If thefrequency of oscillations is ultrasonic, ultrasonic energy is alsoprovided to the tissue, which improves both vaporization andcoagulation, as described above with reference to cutting.

While the preferred volumetrically oscillating plasma flow is the samefor cutting and vaporization, the procedure for performing the taskdiffers. In cutting, the device is held at the surface of the tissue,i.e. 0 mm from the outlet to the tissue. This position maximizes theamount of tissue vaporization and speeds up the cutting process. Invaporization, on the other hand, precise control of the depth ofdestruction is required. FIG. 36 shows that the optimal distance for thetask of vaporization is in region 312. Preferably, plasma-generatingdevice 32 is held at a distance of 2-5 mm from the surface of thetissue. Because the temperature drops rapidly over this region, theoperator has good control of the rate of vaporization by simply movingthe device towards or away from the tissue surface.

FIGS. 48A-E show the vaporization of a tumor according to the preferredembodiment. FIG. 48A shows tumor 371 growing on the surface of healthytissue 370 before the application of the radially oscillating plasmaflow. The goal of the procedure is to completely destroy all of theundesired tissue of tumor 371 while simultaneously coagulating theunderlying tissue. In FIG. 48B, plasma-generating device 372 hasproduced radially oscillating plasma flow 373 and is directed at a partof tumor 371. Plasma-generating device 372 is held between 2-5 mm fromthe surface of tumor 371. The operator adjusts the distance ofplasma-generating device 372 to maintain a controlled and steady rate ofvaporization. While plasma flow 373 is directed at tumor 371, multipleoscillations of the plasma flow occur. During high intensity intervals,high intensity plasma accomplishes both sublimation of tumor tissue andrapid formation of spongy layer 374. During low intensity intervals, thelow intensity plasma also accomplishes sublimation of tumor tissue. Dueto its lower temperature and heat flux, the rate of sublimation achievedby the low intensity plasma is much lower than the rate achieved by thehigh intensity plasma. However, the low intensity plasma is applied for8.5-20 times longer than the high intensity plasma, so significantsublimation may be accomplished even with the lower rate. So substantialsublimation occurs even during the low intensity interval. Besidessublimation, the low intensity plasma also provides heat to the compactlayer through the process of heat diffusion from the fluid boundary.This alternating process of coagulation, where the high and lowintensity plasma work together to efficiently form a sealing layer, wasdescribed above with respect to both coagulation and cutting. By thetime the operator moves plasma-generating device 372 laterally to theposition shown in FIG. 48C, the radially oscillating plasma flow hasvaporized part of tumor 371 and simultaneously formed a sealing layer toprevent bleeding.

FIG. 48C shows tumor 371 after several oscillations of plasma flow 373have been directed at another part of the tumor. Typically, the operatormoves plasma-generating device in a circular motion parallel to thesurface of tissue 370 so that the entire tumor is vaporized at acontrolled and even rate. At this time, the top portion of tumor 371 hasbeen destroyed, but some undesired tissue remains embedded in healthytissue 370. At this point in the procedure, the operator may optionallyremove plasma-generating device 372 and determine if tumor 370 has beencompletely destroyed. One way to determine if the tumor has beencompletely destroyed by visual or tactile examination. If the tumor isstill present, it will have a noticeably different texture than healthytissue 370.

As shown in FIG. 48D, the operator again directs plasma-generatingdevice 372 at tumor 371. To maintain the same rate of vaporization, theplasma-generating device is moved forward slightly, staying within 2-5mm from the surface being vaporized. As a result, more undesired tumortissue is vaporized. FIG. 48D shows that the plasma has vaporized pastthe bottom of tumor 371 and begun vaporizing healthy tissue 370. Becauseof the precise control the operator has over the depth of destruction,the operator will be able to consistently cease vaporization soon afterreaching healthy tissue 370. Additionally, tissue underneath tumor 371coagulated by the radially oscillating plasma flow, so bleeding does notoccur. Vaporizing tissue below the surface of tissue 370 has formed adepression in the tissue which has walls 376. Because of the significantradial component of the dynamic pressure provided by a radiallyoscillating plasma flow, walls 376 will also be efficiently coagulated.

To finish the procedure, the plasma-generating device is again directedat other parts of tumor 371. This movement is shown in FIG. 48E, whereplasma-generating device 372 has been moved substantially parallel tothe surface of tissue 370. Continued application of radially oscillatingplasma flow 373 has completely destroyed tumor 371. All that remains ishealthy tissue 370, along with a sealing layer that completely stopseven very intense bleeding. As explained above with reference tocoagulation and cutting, the sealing layer has a morphology unique tovolumetrically oscillating plasma flows which makes them preferred forstopping high levels of bleeding. Spongy layer 374, formed primarily byhigh velocity, low density plasma during the high intensity interval,has many sublayers corresponding to each high intensity interval. Also,compact layer 375 is relatively thick compared to spongy layer 375. Thisis possible because during the low intensity interval the low intensityplasma had the opportunity to heat the tissue in the compact layerwithout completely destroying the spongy layer formed during the highintensity interval.

The above discussion of vaporization applies to radially oscillatingplasma flows with frequencies greater than 2,000 Hz. If the frequency isultrasonic, the surgical task of vaporization is improved by theadditional ultrasonic energy transferred to the tissue. In the preferredembodiment, the frequency of oscillations is 20,000-30,000 Hz. Thebenefits of ultrasonic energy for vaporization are very similar to thebenefits observed in the surgical task of cutting. Specifically,cavitation at the surface of the tissue causes fragmentation, aiding inthe destruction of the tumor. Cavitation within the tissue also improvesthe structure of the sealing layers. For example, the pore diameters ofthe spongy layer are reduced. Additionally, the compact layer has asublayer composed of homogenized tissue where cavitation has destroyedthe individual cell structure. This homogenized sublayer is particularlyeffective for coagulation. The end result is that the thickness of thesealing layer can be relatively thin without sacrificing the ability tostop very intense bleeding. This is particularly important for thesurgical task of vaporization, where tissue must be destroyed whileminimizing collateral damage.

In addition to coagulating tissue, a radially oscillating plasma flowused for vaporization can also simultaneously seal blood vessels exposedby the vaporization process. FIG. 49A-B show blood vessel 38 beingsealed during vaporization. FIG. 49A shows tissue 370 near the end ofthe vaporization procedure. Blood vessel 378, which provided blood tothe recently vaporized tissue, is now exposed. Unless blood vessel 378is sealed, blood 379 will continue to flow out of the vessel. When suchan exposed vessel is observed, the operator can use the same radiallyoscillating plasma flow used for vaporization to seal the exposedvessel. To do so, the operator directs radially oscillating plasma flow373 at vessel 378. Due to the high dynamic pressure and heat flux,plasma flow 373 is able to evaporate blood 379 and penetrate deep intoblood vessel 378. Because of this, blood vessel walls 377 will be heatedto a depth of 1-1.5 vessel diameters. As walls 377 are heated they swelluntil they completely occlude blood vessel 378, forming a seal to stopblood flow. This is shown in FIG. 49B. While vessel 378 is shown at thebottom of the depression formed by the vaporization procedure, it ispossible that the blood vessel could be exposed along wall 376 of thedepression. Even though vessel 378 is not aligned with the plasma flowaxis in this case, the radially oscillating plasma flow can stillaccomplish vessel sealing. This is due to a significant radial componentof the dynamic pressure of plasma flow 373.

While the above discussion of FIG. 49A-B describes the vessel sealingprocess separately from vaporization, it should be noted that theprocess of vessel sealing is occurring simultaneously with thevaporization process. The operator does not significantly alter thevaporization procedure described in reference to FIGS. 48A-E. At most,the operator may need to spend slightly more time directing plasma flow373 at a particular part of tumor 371 to seal a vessel, beforeproceeding to finish the vaporization procedure for the entire tumor.

In addition, in the preferred embodiment, with a frequency ofoscillations in the range of 20,000-30,000 Hz, the cavitational effectdue to the ultrasonic acoustic waves helps to seal the blood vessels.Experiments show that the thickness of blood vessel walls increase by afactor of 5, compared to the swelling accomplished without ultrasonicwave, facilitating rapid sealing of the vessel.

5.5 Non-Surgical Applications

Volumetrically oscillating plasma flows are also useful for a variety ofnon-surgical applications. These include medical applications outside ofsurgery as well as non-medical applications.

5.5.1 Wound Treatment, Cosmetics, and Pain Management

Volumetrically oscillating plasma flows can be used for non-surgicalmedical applications. For example, these plasma flows may be used inprocedures for wound treatment, pain management, and plastic surgery. Inthese procedures, a volumetrically oscillating plasma flow is directedat a tissue. The goal of the procedure is to accomplish beneficialeffects by generating and delivering certain chemicals deep into thetissue in the presence of heat and light. These chemicals, like NO orozone, may be formed as reactants are introduced into the plasma flow.These reactants may be air, other gases, or conceivably other materials.Once introduced into the plasma flow, these reactants are heated andtake part in chemical reactions which form the desired chemicals. Thesechemicals are then directed at the tissue surface. In theseapplications, the total heat provided to the tissue is preferably low.Therefore, in a preferred embodiment, the volumetrically oscillatingplasma flow directed at the tissue is a volumetrically oscillatingintermittent plasma flow.

A key benefit of using plasma flows to accomplish drug delivery in theseapplications is the high level of penetration of drugs into the tissue.Drug delivery has been shown to be significantly better when plasmaflows were used compared to methods which deliver the drugs ‘cold’ tothe surface of the tissue. By using a volumetrically oscillating plasmaflow rather than a continuous plasma flow, this drug delivery is evenmore efficient. The benefit from volumetrically oscillating plasma flowsis due to the bursts of high intensity plasma which reach highertemperatures than a continuous plasma can without damaging devicecomponents. These increased temperatures decrease the density andincrease the dynamic pressure, which results in improved plasma and drugpenetration.

Ultrasound has been shown to have a synergistic effect with some drugs,called chemopotentiation. Ultrasound effects cell membranes andtemporarily increases their permeability. When combined with therapeuticdrugs, more drug molecules diffuse into cells and produce an enhancedtherapeutic effect. This synergetic effect makes treatment with thesedrugs more effective. In a preferred embodiment, the volumetricallyoscillating plasma flow oscillates with an ultrasonic frequency, i.e.the frequency of oscillations is greater than 20,000 Hz

5.5.2 Plasma Waste Disposal

Volumetrically oscillating plasma flows can be used to efficientlydestroy waste. The use of continuous plasma for waste disposal is wellknown, and the process is called plasma arc gasification. Ingasification, one or more plasma flows are directed at waste inside alarge furnace equipped to capture gaseous and solid byproducts. Theplasma flows with a high temperature cause molecular dissociation in thewaste and turn it into simple chemical components. This process safelydestroys municipal, hazardous, and medical wastes rather than storingthem in landfills. The resulting byproducts of the gasification processinclude valuable reclaimed metals and silicates, fuel and fuelintermediates such as syngas, and chemicals useful for industrialapplications.

Volumetrically oscillating plasma flows and particularly the flowsoscillating at ultrasonic frequencies can be used to improve theefficiency of plasma waste disposal. If oscillating at an ultrasonicfrequency, the plasma flows supply ultrasonic energy to the waste inaddition to the high temperatures of a typical plasma flow. Thisultrasonic energy agitates the waste and fragments it into small pieces.This agitation preferably speeds up the chemical reactions which destroythe waste.

One current problem with plasma arc gasification systems is the rapidbreakdown of components of the plasma waste disposal system exposed tothe high temperatures of the plasma. For example, liners of the plasmafurnaces in typical systems may have a service lifetime of one year orless. The short lifetimes of components increase maintenance costs ofsuch systems. By using a volumetrically oscillating plasma flow,however, the average temperature can be kept relatively low whileproviding bursts of high intensity plasma at very high temperatures.This lower average temperature preferably reduces the strain on systemcomponents and increases their service lifetimes.

FIG. 50 shows an exemplary embodiment of a volumetrically oscillatingplasma waste disposal system. Waste 401 enters furnace 402 through wasteinlet 403. Plasma-generating devices 404 provide volumetricallyoscillating plasma flows 405 which during operation destroy waste 401.Slag 406 comprised of molten solid byproduct accumulates at the bottomof furnace 402 and is collected for further processing. Gas byproductsare captured by vent system 407.

In some embodiments plasma-generating devices 404 may have dimensionsmuch larger than the preferred dimensions for embodiments adopted formedical applications. For large scale waste processing, current levels,plasma-generating gas flow rates, and diameters of various portions ofthe heating channel may all be many times larger than the correspondingdimension of surgical embodiments described above.

5.5.3 Plasma Cleaning

Volumetrically oscillating plasma flows can also be used in improvedsystems and methods for plasma cleaning. In preferred embodiments,components of a plasma interact with a surface in several ways to removecontaminants and create an ultra-clean surface. First, the neutralplasma-generating gas atoms are directed with a high velocity at thesurface being treated, removing and vaporizing contaminants. Second,other gases introduced into the plasma flow, such as oxygen, becomeenergized and react with contaminants to form new compounds which areeasily be removed from the surface. Third, photons generated by theplasma flow break down molecular bonds in the contaminants, facilitatingtheir removal. For example, high molecular weight organic contaminantscan be removed from a semiconductor substrate to create an ultra-cleansurface appropriate for electronics fabrication.

Volumetrically oscillating plasma flows oscillating at an ultrasonicfrequencies provide additional ultrasonic energy which preferablyimproves the cleaning process. Similar to the action of a sonicator, theaddition of ultrasonic energy agitates the surface being treated andimproves the efficiency of reactants in removing tightly adhering orembedded particles.

6 EQUIVALENTS

Many modifications and variations of this invention can be made withoutdeparting from its spirit and scope, as will be apparent to thoseskilled in the art. The specific embodiments described herein areoffered by way of example only, and the invention is to be limited onlyby the terms of the appended claims, along with the full scope ofequivalents to which such claims are entitled.

What is claimed:
 1. A system comprising: a. a power supply for generating a current wave having a low current level and pulses reaching a high current level according to a controlled pattern; and b. a plasma-generating device configured to: i. heat a plasma-generating gas to a first temperature with the current wave at the low current level, wherein the first temperature is at least 10,000 K, ii. heat the plasma-generating gas to a second temperature with the current wave at the high current level, wherein the second temperature is at least 10,000 K above the first temperature; and iii. discharge from an outlet the plasma-generating gas as a plasma flow that alternates between a high intensity plasma flow and a low intensity plasma flow according to the controlled pattern.
 2. The system of claim 1, wherein the plasma-generating device comprises an anode forming a portion of a plasma channel, the plasma channel having the outlet with a diameter of 0.3-0.8 mm.
 3. The system of claim 1, wherein the high current level is less than 50 A.
 4. The system of claim 1, wherein the plasma-generating device is further configured to receive the plasma-generating gas at a flow rate of 0.1-0.6 L/min and at room temperature.
 5. The system of claim 4, further comprising a control circuitry for monitoring the flow rate of the plasma-generating gas.
 6. The system of claim 1, wherein the outlet is a first outlet, and the plasma-generating device comprises a cooling channel with a second outlet near the first outlet and for discharging a coolant.
 7. The system of claim 1, wherein the plasma-generating device comprises a cathode assembly comprising multiple cathodes.
 8. The system of claim 1, further comprising a control circuitry for causing the power supply to generate the current wave.
 9. The system of claim 1, wherein: the current wave is a biased pulse wave with a pulse frequency of 20-100 Hz and a duty cycle of 0.05-0.15, and the low current level is in a range of 3-10 A and the high current level is in a range of 25-30 A.
 10. The system of claim 1, wherein: the current wave is a biased pulse wave with a pulse frequency of above 2,000 Hz and a duty cycle of 0.05-0.15, and the low current level is in a range of 3-10 A and the high current level is in a range of 25-30 A.
 11. The system of claim 1, wherein: the current wave is a modulated biased pulse wave produced by modulating a first biased pulse wave with a pulse frequency of above 20,000 Hz and a duty cycle of 0.35-0.65 with a second biased pulse wave with a pulse frequency of 20-100 Hz and a duty cycle of 0.05-0.15, and the low current level is in a range of 3-10 A and the high current level is in a range of 25-30 A.
 12. The system of claim 1, wherein the current wave is a modulated biased pulse wave, in which a high frequency biased pulse wave is modulated by a low frequency biased pulse wave.
 13. The system of claim 1, further comprising a processor and a user interface for programming a mode of operation of the system.
 14. The system of claim 1, further comprising: a console including the power supply; and a connector to the plasma-generating device for providing to the plasma-generating device energy in the form of an electric current.
 15. The system of claim 14, wherein the connector further supplies to the plasma-generating device the plasma-generating gas.
 16. The system of claim 1, wherein the plasma flow has an active zone defined by plasma having a temperature above a threshold, the active zone expanding and contracting volumetrically upon discharge from the plasma-generating device according to a controlled pattern.
 17. The system of claim 1, wherein the plasma-generating device is a hand-held surgical device.
 18. The system of claim 1, wherein the plasma-generating device comprises an anode, a cathode, and a heating portion, and the plasma flow is formed with an electric current arc established between the anode and the cathode.
 19. The system of claim 18, wherein a temperature of the plasma flow is proportional to a ratio of a current of the electric current arc to a diameter of the heating portion.
 20. The system of claim 18, wherein the pulses have a ramp rate of at least 25 A per 10 μs. 